I agree with everything except #5. Here's why:

I create a custom game. My friend joins said game. We start. I leave. He is dropped and receives 250 favor. He creates a custom game. I join. He starts. He leaves. I receive 250 favor.

Rinse and repeat.

Points one and two of the OP, I figure, should be a given.  I'm still surprised that those even need to be discussed.

I'm not a big fan of the random dropping of a player from the other team, though.  Yes, it evens things up but I don't see the merit in punishing someone from the team who decided to stick it out.  If there's a statistical way to balance the scales as IV suggests, I'd much prefer that.

That's for custom games, though.  In a truly competitive setting, I wouldn't advocate any special boons.  If someone on the team bails, that's it.  No added bonus - you fight with what you have or concede the win (without forcing the winning team to sit through a boring AI match). 

Yeah I know, but they get a free win and favor + the remaining players still get to not have a wasted game on their hands. in a 5v5 that would be 8 players benefiting. You'd never be less than a 50% favorite to benefit from it and you'd often be more.

What do you mean?  I've forever supported the solutions I posted.  Coincidentally - they're the same things that happen in Dota (in Dota you could take the leaver items, but I think you should just get the gold here)

I know it works because I've won games of Dota even with people having left on my own team.

Yes yes, I said retarded. Is it really necessary to call people immature over using a widely used slang for 'your idea is dumb and by extension I will call you dumb as well. Congratulations'. Ps. Your idea was really dumb. 

There is a substantial difference between someone leaving and (lets call the effect worsening the game), where you can decide between continuing the fight (this happens quite alot. especially in 4v4 and more), leaving yourself, or deciding to remake the game or join a new one vs. being forcefully booted out into the lobby, feeling like you got screwed out a game you wanted to be in. Maybe your friends are still in the game and you have to wait for them to finish, or have them leave and get a loss. In this case I consider letting one side have the number advantage to be a better solution.

That's not to say that if the current trend of ragequitting causing an unravelling of the game isn't equally bad design (I'll call it design even though it was obviously unintended), but your point 4 is like giving a car with quadratic wheels triangular wheels and pretending it's fixed.

Also split the continuous gold they would have been getting among the other people.

That's a lot of gold.

Don't you think at some point that tips the scale the other way? Wouldn't it make you disadvantaged if a teammate did not leave? I mean everyone wants more gold. I know I want more gold. I just don't think that really addresses the problem, particularly in a 2v1 situation.

Sure, it would help. But it isn't going to make up for the fact that you can't be two places at once and flag control is important in this. DotA's only similarity would be barracks. 2v1 imba is effectively like losing a barracks automatically.

No?  Each team would get X gold/second, regardless of the number of players.  No imbalance there.  The hero kills from being down a man will probably make the difference, but each hero on the leaver side would have better items thanks to the gold redistribution and such they might have a chance.

This one is not bad. It is not as good... but it is not bad.

You have too much time on your hands. Public ladders mean nothing and have meant nothing for every RTS ever made. Private ladders are the only things that show true ranking because the community is the only entity that cares about the success of a game after initial sales. GPG doesn't make money off people playing their game once they've already bought it so why spend money making the ladder system actually matter?

In house, private league, private ladder, and tournaments are the way to go if you want to avoid all the annoying things that make the game difficult to enjoy.

Aye. This (or a variation such as giving only 10% of the gold) is suggested in about every thread like this, but still most people seem to insist that kicking the DG wholly out is a better choice. They never explain their reasoning behind it, though.

I don't know... Despite being easier to kill than most people, the AI still gains gold, experience, and puts up a decent fight. 

It's hard to say what would really make for a 'balanced' match between the arbitrary mechanics being described in this thread but I don't like the thought of adding a DG to the mix that runs around with little to no cost in dropping dead.