If you're on a win streak solo...

I don't think so. That would be a lot of extra work for Riot that doesn't result in substantial (if any) gain.

no

Clearly that doesn't make sense. If they paired up win-streaks and lose-streaks at least one of those would continue on their respective streaks. If anything, to maintain a 50-50 winrate they should pair up those on win-streaks and give them stronger opponents to compensate. Oh wait. That's exactly how MMR works.

well apparenly for me and dozens of ppl on my friendlist that happens. You cant have decent people for a 2-3 days and the suddlently everyone you match is trash considering your mmr grew better due to the wins.

Yes, it does. The system is made to try to keep you to as close to a 50% winrate as possible, so if you start pulling away from that, prepare to carry hard.