] At the beginning of the last chapter of her last book, Jane writes "When any two young people take it into their heads to marry, they are pretty sure by perseverance to carry their point, be they ever so poor, or ever so imprudent,...This may be bad morality to conclude with, but I believe it to be truth;...". This suggests that Tom and Jane were just good friends?

Frank, IMO Jane and Tom had a lovely intense and brief flirtation, definitely more than "just friends." Tom's relatives were upset because they thought Tom was giving Jane the impression that he would "make an offer" when his finances and his planned future at the bar would make that most imprudent. So they sent him away rapidly, and she never saw him again.

The key point in your quote above, I think, is "...take it into their heads to marry..." I'm sure they never reached this point of talking of marriage, just having fun dancing and talking together (of books like "Tom Jones" and many other things...). I imagine that Tom was a very intelligent and delightful character, and Jane never met anyone quite like him later.