Any advice to deal with unfairness and/or toxicity in the game?

For the post-game lobby, drop a GG in it and hide it. There is no point in letting them tilt you after you just got your @$$ beat. Just appear a good sportsman and let it go.

In the end, good survivors will make a monkey out of a killer. You can watch the jokes of tournaments that have been hosted for DBD to know that. Let defeat be your default condition you're happy with, and savor the wins. You aren't supposed to win in the first place.

Keep your mental clean. Most survivors who go out of their way to be toxic are revealing a weakness of some kind. Clicky flashlight people who want to make you chase them will follow you and do no gens. If you kill their team instead, they're only wasting their own time. If you take a chase in a super strong part of the map and only realize it when the damage has been done, don't kick yourself over it. Take the lesson that "Hey, this game has a broken spawn here, next time I need to know that Suffocation Pit spawning alternating TL and long wall window gyms is stupid broken and not chase here ever!".

For flashlight saves, always make sure you face a wall straight-on in order to deny most saves. BHVR did something weird to flashlights recently - sometimes you get blinded through a wall or at a 180degree angle, but for the most part facing a wall helps. If you know you have someone hovering around you and waiting to try a save, you can stand still over a downed survivor and look down to fake the pickup, luring the savior out of hiding and maybe even getting a free hit on them. From there you can just chase the savior and leave the first survivor slugged, generating double the pressure for the price of one.

Bodyblocking can be nasty, but map permitting I will spin on my heel and go to a different hook. If you really aren't going to make it to a hook, just take the free protection hits they give you, drop your survivor as a slug and then begin chase anew. You're gaining more pressure than you are losing in most cases.

For Keys, there is very little counterplay. 1 gen left is the part of the game where the killer should be strongest via a 3gen strat, as survivors have the least resources and need to play in a more concentrated area of the map. Keys allow them to completely avoid the killer's strongest point in the game. I personally think that Keys and the Hatch in general are just terrible gameplay mechanics, and I can't really tell you much except either tunnel whoever is holding the key down first, and get used to losing anyways. You can't fight the game, only play within its constraints. This game has a free RNG escape for survivors.