You can't do any of that without agitation. You will just burn out while everyone else takes credit for what you're trying to take credit for. This is the usual course of events for Western socialists bumbling into union organizing.

You also cannot provide uncritical support externally and expect to do anything but sharpen the knives that will be used against you by the people that you leave in power there through a lack of organizing and agitation. No socialist program has ever been won through uncritical cheerleading of unions. Unions have petty bourgeois characteristics that must be constantly combatted so that they can become tools for class struggle rather than class collaboration.

Re: internal unity, there is no such thing if you also want socialists in leadership positions. That is going to necessarily create disunity because your socialists will be advocating for something different than the liberals, proto-fash, and staff, and they will all fight you over it. You can support unity of action when it comes to direct actions but if you prioritize unity over agitation then you will have to pretend to be liberals and act like liberals the entire time like some kind of underground operation. There are not enough of us for that level of salting to ever work. There has never been.

Re: alienation from external criticism, this just means you have to be competent in your criticism. It needs to serve building an org within the union, the faction you will have to create if you ever want to succeed. Many Western leftists have no idea what they are doing and are not competent about it, so they do just alienate workers. But that's a reason to become competent, not throw out the tool that has worked time and time again.