Rafting Renewal - KR Update. |:

Just how much Expl EXP do dead hobgoblins give?

Not much per but enough to always give more exp than it would now.

wait...do they only give Exploration? or is Normal exp given too?

Probably both. Metus inhabitants and Iria field bosses give both normal and Expl EXP for killing them.

If the stars takes form of a sort of gold items, then does that mean you can stack them from repeated rafting now? Or do they still all vanish if you enter another raft?

Yes, they stack up to 100 per slot and expire after 3 days (if they're like book pages, expiration time is extended by mixing stacks of older stars with more recent stars). If they didn't stack and disappeared after reentering rafting (which they do not), there would be no point to this update.