Q: When you’re grappled you don’t lose your active defense against the one grappling you. What about when you’re pinned? Since your opponent has greater control over you...

You keep your active defense against the character pinning you, because they are distracted by the need to keep you pinned. Similarly, they don’t get the +4 to hit you (per page 198). They do get other advantages, also described on page 198.

Q: When you grab an armed opponent into a grapple, what happens to his weapons? Are they automatically dropped? If not, how hard is it to keep hold of them? Do they impose a penalty on his grappling check? If so, is it the same for all weapons (imagine trying to grapple while keeping hold of a greataxe)?

A grappled character keeps hold of his weapons unless he chooses to drop them. He doesn't need to make a check to keep them, nor does he suffer a grapple check penalty for the weapons he is holding, regardless of what they are. Note however that he can’t use any weapon but an unarmed strike or light weapon to make attacks while he is grappled, so he may well choose to drop larger weapons.

Q: Assuming the opponent never escapes, how long can you lift and hold a grappled opponent?

Refer to Carrying Capacity.

Q: If you pin an opponent and no 3rd party is in the grapple, isn't Release better than Escape from Grapple?

Yes. In point of fact, you would only need to use Escape if there was a 3rd party in the grapple.

Constriction Damage

See Constrict for details.