Anyone have experience with Flair Airlines? I'm considering flying them this summer, but had some questions about seating:

1. if I opt not to pay for a seat in advance, I understand they will assign me a seat at check-in (or do I get to choose my own?). After check-in would I then be able to change to another standard economy seat for free (like Delta)? Or am I stuck with what they assign unless I pay?
2. if I opt not to pay and there are other empty seats once on board, am I allowed to move to a vacant coach seat?
3. If I do decide to pay for a seat, but wait until after purchase to add it, is the price dramatically more (I would do this so that I can claim the seat assignment as an incidental credit, rather than having it lumped into the ticket purchase).
4. re: personal item - for a short trip I'd be fine taking just a personal item, but most of my backpacks seem a bit bigger than the published limit. I would not plan to stuff it full. If my backpack happens to be slightly longer than the limit but isn't full are they extremely picky? I am going to guess that this is a YMMV situation, and most likely will go to great efforts to find something that stays within their published limits, but thought any personal experience might help.

Thanks in advance for any advice you can offer.