1) Since enthusiasm moves back to 5 slowly, there is a de facto cap of about 13. However, that is only maintainable when playing TIE three times a week. After dropping out of the cup, 10 is the maximum.
2) This question is probably too unspecific. Also, I don't know exactly which sub level you mean. The potential soft cap is still not fully understood though.
3) Generally, yes. However, Stamina 10 and GS 9 do not.