Just say you can not be arsed to be a vegan, even though you know you should.

But that's not really how it works in practice, is it? We don't limit meat production to what can be produced as a byproduct of ecological management, or scavaged after an animal's natural death. The material conditions that actually exists are that there is a massive industry that's exploitative of humans and animals alike, and it's causing tons of needless cruelty and wanton ecological damage in order to provide people with meat.

Even if you limited yourself to only eating meat produced in the ways that you described (plenty of people will deploy this argument and then make no attempt to actually live up to the standards they put forth, but I'll give you the benefit of the doubt), would it not be better to sell the meat produced this way, so that some other carnist will eat that rather than meat produced from a factory farm?

This is an idealist argument. Just because you can engineer some hypothetical situation where eating meat doesn't cause harm doesn't mean that that hypothetical is relevant to the vast majority of cases that actually exist.