Not all of them. Different Buffy comics have various storylines and timings. Some could fill in gaps from earlier seasons or offer alternate perspectives rather than strictly following after Season 7.
Buffy's relationship with Spike in the comics varies. Sometimes they have significant interactions and may even be romantically involved, but it's not a constant throughout all the comic series.
The answer varies. Some Buffy comics are explicitly tied into the main continuity and are considered canon, but there are others that might be more standalone or not as directly related to the core story, so their canonicity is up for debate among fans.
It varies from comic to comic. Sometimes they have a romantic connection, while in others, their relationship is more complicated or not existent at all.
Buffy's age in the comics varies depending on the specific storyline and continuity. But typically, she's depicted as being in her late teens or early twenties.