1. some of the concepts glossed over so far are included IN DEPTH in "STICK AND RUDDER", including an exercise you can do at home which just amounts to standing on your toes and then coming down on your heals.
2. Seat height is vital, so vital that most airliners have EYE AIMERS to ensure that your adjust your seat just right.
3. depending on the plane, you could sit in the pilot seat and have your instructor hold the tail down so you can, at your leisure, see the flare or landing attitude (engine off, no money!).
4. one of the best exercises, with instructor, is to fly the length of the runway without landing, just above touchdown...do this a few times to a go-around...and then, just when everything looks right, the instructor can cut the throttle and you end up landing quite nicely.
5. judging flare height is covered in "stick and rudder" using everyday objects to actually judge height.
6. I am assuming you are a boy/man...and you should know what six inches is...men have been lying to women for many years about what six inches is and that is why women have been making such bad landings . ;-)
7. mentioning things like tire skids on runway to judge height is fine, and guess what, if you use a landing light at night, it works then too!
It takes a long time to learn how to land. Try going out to the instructor bench and sit there and watch others land. if you are not damaging the plane, you aren't doing too badly.
using multiple instructors isn't a good idea in my view...we don't do that out where I teach/taught.