I think the combos will mostly be relevant when it comes to soloing (not so much the ones that are position related) and for bosses, since they can actually last long enough in a party environment to not explode. Most parties I feel are still going to kind of run like burn parties and people will just be spamming abilities.

I don't think the time between abilities for a combo will persist indefinitely. I think it was even pointed out in a translated post (don't have the source handy, sorry) that it would be something like 10 seconds before the next ability becomes invalid.

I also don't believe battles will be much more fast paced than before since it was mentioned in the patch notes that TP gain will be slowed down to accommodate for TP not being expended from the combo moves.