It depends. Some Star Trek comics are officially considered canon, while others are not. It often depends on the specific storyline and its approval by the franchise's creators.
It depends. Some rare or highly sought-after Star Trek comics can be quite valuable, especially if they're in good condition and have significant collector interest.
The status of IDW Star Trek comics as canon is a bit complicated. Some are officially recognized as part of the Star Trek universe's continuity, while others are more standalone or alternate takes that don't strictly fit into the main canon.
Not all Star Trek comics delve into Guinan's backstory. It really varies from one comic to another. Some might touch on it briefly, while others might not at all.