This is all hypothetical. These are hypothetical houses and a hypothetical owner in a hypothetical market, frozen in time. There is a direct correlation between quantity and quality, since changes in quantity lead to changes in quality. This is a fundamental part of socialist theory of change. This is not a moral consideration, there is no "right amount." Because there are absolutely no possible scenarios where someone owns 10 vacant houses and just loses money on them indefinitely. Is this someone who owns and maintains ten houses all for their own personal recreation and use? How can they afford 10 houses?

The discussion is about the difference between personal and private property. The definitions of these things are socially contingent. We cannot conduct analysis outside of the social context and be serious about truth or social transformation. There is no arbitrary number cut off, splitting this hair in this way is completely besides the point. There is no reason for someone to own and maintain 10 empty properties while others have nowhere to live. My point is even if we aren't consciously capitalizing on the houses, the houses exist within a context. Broadly, that's capitalism, and broadly the properties still function as part of capitalism whether the owner intends it or not. Trying to further abstract this hypothetical in a desperate search for a generalization that is so alienated from reality that you "might be" sort of correct if viewing the situation from one very narrow perspective does not help anyone understand the relationship between personal property and private capital. Because over time, at some point, circumstances beyond our ideals decide that our ten houses are worth more if they have a use value or an exchange value for us. The system asserts itself upon our lives, and there's no real escape.

You have not discovered a loop hole by inventing a kind of property that has neither a use value or an exchange value. You've merely discovered a pit of idealism where material relations exist to suit one's own confusion and wrong headedness. This is not a road to understanding socialism. But I do appreciate the contribution to discussion.

My house and garden can be a place of production (for art, artisan goods, handicraft, foods, etc.).

That is not "letting the owner live off other people's labor" it's letting the owner live off their own labor. Do other people have to come over to your house and garden to make things? They should have their own house and garden. If they then want to come over to hang out and make crafts, then what would possibly be the problem?

May a musician own musical instruments or must he collect them from the warehouse of the local musical collective?

Again, so long as a type of tool is accessible to anyone, there's no issue with someone owning their own set. Whether it's a musician with an instrument or a plumber or a mechanic that owns their own tools, there's not really a problem. Nobody's going around confiscating everyone's wrenches for some ideological reason.

Suppose there's some very expensive instrument, say a massive pipe organ in some church. If that organ is not being used for profit, then it doesn't matter, but if it is being used that way, then perhaps it would make sense if various musical groups were allowed to use it occasionally, since it might not be practical to ensure everyone has their own pipe organ.

Computers and other electronics are classical dual-use goods. Do I have to share them with all kinds of people?

In a modern economy where everyone has their own computer, that's completely unnecessary. Suppose there's some remote village somewhere where they only have one computer. In that case, surely it makes sense for access to be shared.

Cars and other vehicles.

Repeating myself a lot here. Socialists aim to make transportation accessible to everyone, which generally means investing in public transit. But there are cases where it might make more sense to make it easier for everyone to have a car, like a remote region where public transportation is less viable, or a situation where people already own cars. I think most socialists would favor a gradual phasing out of cars in favor of public transit for efficiency, accessibility, and environmental reasons. A socialist state that doesn't already have cars probably wouldn't prioritize them, but that depends on the material conditions (as I said about rural regions).

The distinction is a class distinction. Capitalism functions to.ensure that the greatest number of people do not own enough to be self-sufficient. Peasants under feudalism were self sufficient and communal, so the first job of the capitalists in countries that were industrializing was to buy up most/all the communal land, kick everybody off of it and force people to still work it for a paltry wage. There were still people who owned houses and land during this period, but legal and economic pressures forced people to give up what they owned and get a waged job. Even today, small family owned farms fall into debt and get bought up by large landowning companies/trusts and converted into farmland or data centers. It isn't happening to everyone at once, but its a gradual process where it happens to a few people here and there. Some people buy farms and are successful, there are always contradictions and exceptions.

If you are able to sustain yourself 100% on your own production, that's extremely rare. My cousin was a nurse practitioner and stacked cash his entire life, and in the last few years became like a yeoman. He butchers his own meat, he grows much of his own food, but still buys 10-20% of what he needs, mostly flour and grains. Under socialism, at least any socialism that I would fight for, he would be left alone, because he really isn't the problem. The family that owns an ice cream shop or a bakery, particularly small businesses who help people, contribute to social movements, are against ice are pro Palestine, pay their workers well, these people own capital but they aren't the problem.

The social relations you are describing are so far in the future that they are virtually impossible to understand. The first socialist revolution will shift power from the capitalists to the workers, but will not abolish private property. This will begin a phase of transition where private property will be collectivised. But the biggest barrier to collectivization isn't just who owns what, it is the divisions in the working class. There are a lot of social relations that have to be "worked out", and nobody is able to create a one sized fits all solution for everywhere (socialism is necessarily international) at once. Some places might hold on to private property and become more social democrat, retaining their ruling class in order to have higher wages for workers. Other areas might try to become fully communist. Some places will try to ban all private industry, some will collectivise some large industries and leave others privately owned.

Social conditions are deeply contradictory because people are contradictory. So trying to logically work these things out is always gonna be incomplete and imperfect. What matters is the working class continually working together to create better conditions for more people. Eventually, this collective effort will reach like a tipping point, and the next revolution will occur, which will be more humane, more democratic, and less violent than any that came before.

There is a ton of real world struggles that can actually be won (universal healthcare, universal jobs guarantee, robust social safety net, improved transportation/sanitation/education) that can be accomplished by just taxing the rich to the hilt. But in order to get there, we need solidarity and class unity. Once we have that, we can reorganize society in ways where the answers to your questions will have clearer solutions.

Socialism isn't just a future, it is being able to see the resent society, the people in it and their needs and experiences, and working for a better society. IMO individual private/personal property questions will make more sense once the conditions for them have a concrete basis.

If you're hiring then you are own the means of production that should be shared by the workers.