Riot will never touch nor try to fix the 4v5 to be favored on the 4. Why?
The moment they do anything to touch that, they're agreeing that its a good idea to afk in your games and let it be a 4v5 for the fact that the 4, who could be way more potentially better than the 5, can easily decimate thanks to hidden buffs. That's on the gameplay design ideology.
For the LP thing; there's really nothing that could be done to prevent this. I can easily see it being abusable where if a game is going sour, the losing team could easily afk a player to abuse this said strat and force the enemies to lose out on LP gain, whether fairly or not. And, sadly, that would require human interventation at that point, as I don't think its possible for the servers to distinguish whether something happened aggravatedly (that's a word, dammit), or whether the afk was genuine. It's sadly a binary system of sorts.
I still support the idea that the enemy team can issue a mercy to retract an LP loss AFTER the game is over. That would be both logical and balanced, and then make it so all 5 have the right to do that. Then again, even that has flaws with potential high-ranked picks/players mercying players they fight just on the hopes that they stay up there to fight again (pub-stomping and whatnot, as the slang goes).
Ultimately, if Riot is going to touch on this ever, it's probably gonna be after they set the new client in motion for it, which as we all know will happen in Valve time... Where's the clock again?