I’m personally more of a “learn by doing” kind of guy. I don’t think spending long hours in training mode is the best way to learn things - go into the lab, get down the execution for what you want to be able to do (I say 10 times in a row without failing is sufficient), and then go play real matches against real people.
Depending on what you’re trying to train, there isn’t really a “best” way to go about it though. Training mode is good for building the basic fundamentals of a particular action or sequence, Ranked is great for building adaptability to myriad playstyles, and long sets are good for learning particular MU’s in-depth. And different people learn in different ways - some people avoid practice mode generally, and others are serious lab rats. Kind of depends on what works for you.
Infil’s guide does a decent job of laying out the general playstyles of the cast, but for something more in-depth you’ll probably need to connect with the mains for particular characters. Some of the cast members have pretty active sub-forums here on the site (Kan-Ra and Hisako), while for others you may need to look elsewhere. I think each sub-forum though probably has a character specialist or two you can talk shop with though - a lot of times those people are just waiting for someone to ask the questions.
Doesn’t exist to my knowledge, at least not in a comprehensive way. Some of the character sub-forums have some of this info though, particularly for good frame traps and mix-ups. I also believe someone from the forums was trying to detail out all the frame data for the entire cast in the game. That process is time-consuming though, so not sure where they are on it.
You probably need to be a bit more specific on what you’re looking for here. KI has a lot of “advanced” tech, and much of it has pretty unique applications across the cast even if it’s just based on a “simple” idea like resets or 50/50’s.
One of the few times where I do suggest just hitting the lab. Go into practice and work on a particular manual or string of manuals, and then once you’re able to hit it 7-10 times consistently without failing, go into Ranked or Exhibition and practice using them in real setting. There really isn’t too much substitute though for that initial training period in Practice - if you want to learn manuals that is probably where you’re going to do it.
By punching them in the face when they try to push buttons while you’re plus. In most cases, it’s actually quite preferable if some guy doesn’t want to accept the fact that you’re +3 and it’s not his turn yet - it’s a free counter hit into combo for you if he insists on doing stuff.
I believe that being able to shadow counter effectively is probably the most under-used and under-appreciated aspect of defense in this game. Proving that you can consistently shadow counter someone will force them to chill out a bit in their offense, and instead go for stuttered or more committal pressure, where they’re leaving gaps in their buttons or going for throws and such. Offense is crazy strong in this game, but effective use of shadow counters can make things a lot more manageable for you.
If you run into someone in Ranked (or Exhibition) whose play impresses you, just shoot them a message and say “GG’s. Would you mind running a set/FT5/FT10 with me?” In my experience, you’ll rarely have someone say no - even if they can’t at that moment, most of the time they’ll be up for it at some later point.
To start off with though, I’d be happy to play you - I make no promises that I’m better than you, but I’m sure it can be a learning experience 
And as someone else said, the “GG’s Thread” is also a great place to organize sets with people who play a wide variety of characters.
