I think everyone is exaggerating here.It's true that summoner's rift is flooded with smurfs from lvl.5-15 but I think after that it evens out cause people who were playing bots up till level 15-20 decide to start playing on SR. Mind you my smurf is on EUNE so I can play with some friends(my main is in EUW).I play with my friend who is really bad at this game and she feeds a lot,or builds the wrong items.She will get flamed at times but for the most part most of our team(and enemy team) mind their business.She's lvl.20 I think and I just hit 29 so the amount of smurfs might have evened out.
As someone who has been playing since 2011 I've got to say the toxicity is at its lowest at the moment.So people here saying toxicity here toxicity there shouldn't exaggerate.It happens but hardly as much as it used to. I personally don't find anything wrong with smurfs as long as they are there for the right reasons,I made mine to play on a different server and learn so I don't feel pressured to perform well.I understand some people play to make themselves feel better but you meet most of them between lvl.5-15.After that you meet them less I think.
People have offered some good advice like play with friends and I think you should follow that,it helps a bunch.It helped me a lot when I started playing and I was really bad,because it made the game more fun.Playing alone can get a bit frustrating even now,not necessarily cause people might flame you and stuff but because you have better synergy with friends than strangers so you feel safer.
I would also advise you to not bother "chatting" with people on enemy team.If they get the sign that you are bothered by getting camped,taunted etc.They will just do that even more(even regarding ranked people constantly advise that if the enemy whines about getting camped,or whines after dying you camp them more).So don't bother with the enemy unless they're friendly.
Most of the problems you mentioned you'll unfortunately have to deal with them later on too(sometimes in ranked too).Oh you claimed adc?Well maybe you support wanted ADC so you'll get a Miss Fortune to support you(has happened to me).Unfortunately those things will happen sometimes.