These are my personal opinions and not facts but I feel strongly enough about them to defend them. Deserve is a loaded word like TJ had said. I will still use it though because this is opinion based.
How much appreciation and support (if any) do fangame creators deserve, and are they currently getting enough?
I don't believe creators deserve any appreciation or support but I do believe that they should be recognized for the work they had put into creating their finished product based on their ability to create a unique and interesting experience for the player to enjoy.
Is it okay to heavily criticize people’s fangames that they made on their own time for everyone to enjoy?
I don't think its right to heavily criticize people, especially if the creator had not asked for it. I find its not helpful to have random_person#244 make a comment on the pixel placement of every individual spike on your game. Part of being a creator is to be able to read the criticizer just as much as the critique itself. If the person who comment on it has little experience and very little knowledge about the entire scheme of things their feedback is usually unimportant and otherwise not worth the time to consider.
How much patience and forgiveness (if any) should fangame streamers and players be expected to have?
I think people who stream fangames should have some patience and forgiveness but to say how much is an entirely another story. I think its fair to say that if you simply just do not like the game you should not play it but its hard to say how patient and forgiving a streamer should be considering how a lot of fangames are very much the same and you can usually take a good guess as to what will come next and if its worth your investment to play.
What is the right balance of encouraging quality fangames while not scaring off potentially good creators?
There is no good balance for this and there will never be. There are people who are make their first fangame and get entirely destroyed by this community. I've seen it several times where its shoved down their throat about how bad they are which bothers me a lot. Potentially good creators aren't good creators right now and so their ability to judge and criticize the critique itself isn't there yet. As it stands right now I believe a lot of the community is extremely harsh in their critique.
To what extent can you be proud of a fangame that heavily uses other people’s content? (spook jam, medleys, and in general using music+sprites+sounds made by others)
As proud as you want to be about it. There isn't any issue being proud with making a game like this. However, I do think that if you don't build off of the creativity of others its not as rewarding or deserving of recognition or respect. As an example I think Spook Jam is unique and creative in its own right and is not the creation of just one person but many.
Is it okay to use the self-made assets of other people’s fangames?
I do not feel this is appropriate and it bothers me a lot even though its not my own. It makes me lose respect for the people who do this.
To what extent (if any) are quality fangames not getting attention they deserve?
There are fangames that I think are quality fangames and there are fangames others think are quality fangames. There is a very large difference of opinion on something like this but I do think there are many good fangames that don't get the attention they deserve.
To what extent (if any) are fangame creators limiting their creative growth as opposed to making freeware games for a bigger, more general audience?
I think after making several fangames of the same type you limit creative growth as a gamemaker, but not as a fangame creator.