| Everything John said is right on. I'd like to add some from my recent experiences too..
I think the thumb bent/unbent issue will never be resolved, some like it one way, some the other.
I used to play unbent and i switched to bent after a lesson with a good teacher back in January.. here's why:
For me playing with my thumb bent makes keeping my other fingers curved easier (Fingers curved is a must, but it can be done with a straight thumb too). Also when the thumb is bent, the tip rests on the neck instead of pad. The tip has a lot more nerve endings in it and makes it easier to 'feel' the neck, for shifts and such. I don't it's something you conscious notice, but I think it's helped me.
As far as the tightness in your hand, it's most likely due to not having your bass balanced well. Sitting down may help prevent this but i don't think it's a good 'fix' you should be able to play standing up too. I had problems with thumb pain and a teacher (same as above) informed i've been holding my bass wrong for the last 3.5 years i've been studying bass at a music school. I had my bass really high and i had it lean into me a lot. Setup like this, i NEEDED to use my thumb to hold the bass up.
The solution for me was lowering the bass so the nut is about level with my eyebrow (I'm about 5' 10" i keep my endpin at about 2" or so). Then you can use your left leg behind the lower bout of the bass for more support.
I'm not totally comfortable playing like this yet, but it's a BIG improvement.
If your bass is balanced well you should basically be able to take your thumb off the neck and still have no problem fingering notes.
Hope this helps, let us know if you get your problem worked out
-Jeff | 
