On the question what Tigh or the insurgents are trying to accomplish:
I think what they want is the Cylons to leave. If they can't achieve that, they want to set up a situation in where they can flee the planet with the aid of Galactica.
It is true that the Cylons have superior numbers and superior resources. But still, it requires effort to control humanity. The Cylons decided that annihaiting humanity wasn't the right way. But they apparently didn't trust humanity to go their own way, either. So they eventually decided to take control of humanity.
The insurgents on New Caprica attempt to make clear that controlling humanity is not possible. The assumption certainly is that the cylons won't go back to destroying humanity.
It is true that Cylons don't die. But on the other hand, it requires effort to create new Cylon bodies (even if they have millions of them). It is also a painful experience for each individual Cylons. So, each dead Cylon costs material resources and causes pain or trauma.
To control humanity, the Cylons will always need to interact with them, meaning that armed attacks, bombs or suicide bombs can always "reach" them.
The Caprica Police Force is an attempt to reduce the cylon-human contacts. Only trusted humans get anywhere near the bio-Cylons. The rest of interaction is done by these trusted humans. For Tigh and his men this means that they have to make it clear to every human that allying with the Cylons is wrong, even it means killing these traitors.
Suicide Bombings are probably a big scare for the Cylons because they know that the death of a human is permanent. Maybe the cylon god even forbids it explicitely. Anyway, every human willing to kill himself to hurt the Cylons means that he is diametrically opposed to the Cylons goals, and each suicide bombing is a indication of the failure of the (current) Cylons plan.
From the side of Tigh - Suicide Bombings are a good way to get into sensitive areas. It is hard to get a armed force into a enemy building, and you will still suffer losses. They also show clearly to all humans how much fighting the Cylons means to them and might encourage others to aid them (if you are willing to die for something, it must be worth something).
And did the actor for Lee Adama gain weight or was that creative make-up and camera-work?
It's make-up. Jamie Bamber is still as fit as ever, and according to the podcats and blogs on the BSG-side, Apollo will eventually find back to his old shape. Unfortunately, I don't know wether Adama's beard stays or not.
The whole thing with a child by Starbuck isn't some new plot twist, they've been hinting and laying the foundation for it ever since early 2nd season with the episode "The Farm", and since the first season with the Six in Baltar's head implying that the Cylons wanted such a hybrid.
Though to be fair - according to Moore, they didn't know exactly where the "stolen" ovaries would lead to or how they would use it again. But they seemed to have found a way.
The scene with Starbuck and Leoben were quite... disturbing and unreal. "To see the face of god means madness", indeed...
