I sincerely hope we all post better than we play.
You have the hard part down. You understand not only the very difficult theory behind perfect ABC poker but also the tendencies of flawed players and how to exploit their weaknesses and tendencies. This takes a very long time to absorb and the sheer volume of this knowledge cannot be underestimated. Putting it all into practice without error is incredibly difficult because there is so much to remember and you have so little time to make a decision.
Putting what you know into practice is a gradual thing. You get better and better at it the more you play. It's just a matter of speeding up the information analysis process. This requires practice and repitition. As a part-time player (and admitted multi-tabler) you do not play enough to expect to be able process all these extremely contextualized decisions so quickly that you will never make a mistake. Keep heart though, no one plays perfectly, we all make mistakes. The key is just making less mistakes than the competition you are playing against and making less mistakes as you develop as a player and move up in your competition level. I suspect no matter how many mistakes you are currently making you are making far less than your average opponent already.
You will make plenty of mistakes but the important thing to do is to try to recognize those mistakes as soon as possible after they happen. Ask yourself after every hand, "Could I have played that better?" If the answer is "yes" write a quick note down as to what you felt you did wrong. For me I frequently miss river value bets. I know this is a major leak in my game and I am constantly trying to work on recognizing situations in which I have value bets and forcing myself to have the courage to click the "BET" button.
If mutlitabling does not allow you to analyze your play in real time drop tables until you find the right balance. I usually play 4 tables because I know I will make more money for that session, mistakes and all, over just playing one or two tables. I realize that this short-term greed comes at the expense of development of my poker skills but I like to strike a balance. I play the large part of my sessions on multitable auto-pilot and a part of them on one or two tables to work on my game, usually at a higher limit than my home limit.
When I work on my game a go into hyper-paying-attention mode. I'm looking for patterns to develop in the game. I'm looking for unusual moves players make and putting them into context. That 35/12/4 guy in seat 2 C/R the flop with air on a AKT board into a pfr. Later he donks the turn when an A hits and it turns out he has the A. These are things that in time start to say something not only about that specific player but that specific player type. In the future I will be able to exploit these types of tendencies and feel more comfortable in uncomfortable situations when players put me to the test.
I wrote a pretty long post on this in micros a while back that might explain process of finding these patterns better than I could in this post...
Paying attention