firstly, kudos to you for being motivated and asking very intelligent questions. also kudos on making nice art. your sculptures are really fun.
here is my response:
1. many of these topics have been discussed previously on this forum...do some heavy searching on here and you can find the threads and maybe get little pieces of info that you need, and/or ask follow-up questions. an entire book could be written about all the questions youve asked, but time and energy are limited. and besides, this forum isnt the best place for super thorough stuff like that. thats what an in person apprenticeship should provide.
2. that being said, use the info here only as a supplement to your apprenticeship, where you should eventually, hopefully learn about all this from in person sessions with your mentor. all the answers on here will never come close to real life trial and error, especially with some of the questions youre asking which pertain to the more advanced aspects of tattoo pigment manipulation. if youve only just begun tattooing, focus on simple, solid, clean, well-healed tattoos and slowly experiment over time from there with super technical painterly stuff. ALWAYS make sure you get to see healed (and aged!) results.
3. many tattooers take on apprentices, but not many tattooers have extensive knowledge on all the most advanced aspects of art and tattooing, therefore you get a lot of these situations where if a pupil is dedicated, they outgrow their first teacher and need to seek out someone else to teach them new things. there is nothing wrong with that. i simply mention it because of the kind of questions youre asking on here. most if not all of the answers should be taught to you over the course of your apprenticeship because theyre basic enough. in my opinion, anyway. so maybe youre just trying to get a head start on some of the stuff and be prepared (kudos again, if so). but, this brings me back to my first 2 points.