There's definitely improvements that could be made within the system, I completely agree with you OP. Something that could potentially trigger a MUCH faster manual review (such as multiple boots, tears, or a very high number of deaths) I feel could be very useful to finding these players faster.
On the flip-side, I'm somewhat grateful the system is set up the way it is currently in regards to the IFS (Instant Feedback System). If they would immediately flag and ban an account for someone having a potentially bad game or messing up REALLY bad (like face-checking a bush level 1, then getting cheesed in the jungle at the first buff aftewards) there would likely be a large influx of bans as well as posts on the Boards for people genuinely having a poor performance.
I'd love to hear from the community any ideas they have to help this sort of situation out, as it's definitely unfortunately, and I've had my fair share of games where an individual would literally say "time to feed" and then run it down a lane the entire game - only to OP.GG them and notice that they've been doing it for a while and are in a match again after ours is over with. I don't believe you can get your LP back, however the plus side is your MMR will continue to rise or stay the same while theirs will fall drastically - there's MANY players that actually purposely lose a ton of games in a row in an attempt to drop their MMR to abysmal levels and stomp their way through Cardboard 17. It helps a lot of people out to boost their egos, as well as create "click-bait" content if they're after that - being able to boast 40/0/0 scores and stuff of the like, however it isolates a large portion of the community that actually wants to improve and play a team game. It's certainly frustrating to do your best and carry a game as hard as possible pressing every advantage and capitalizing on objectives while CS'ing and warding well only to have a 20/0 Katarina come and say "hello" to you at 10 minutes with a 10k gold lead on everyone in the team. It's demoralizing for a lot of players, and I'm sure it's a topic that can be discussed and improved upon.
Thanks for your post OP, hopefully the community is able to share their ideas and contribute some sorts of solutions to the issue, as trolling is always a "hot topic" and it's very difficult to differentiate someone that's having a bad game and someone that's intentionally ruining the experience for others (from a computer's perspective, at least). Humans and (most) animals are pretty keen to pick up on it quickly, although the amount of manpower involved would be absolutely ridiculous considering the 100+ million players every month.