Whew! Lots of questions!
Is it right that there is a 10 kg limit for luggage and is it possible to pay for extra luggage? - Yes, 10kg limit, but it is possible to pay for extra luggage (11,000 Rp/kg I believe, and I flew back with someone who travelled with dive gear with no problems). I rented my gear from BlueMotion. How was the Laguna? - see my previous description, I was comfortable there. Originally I planned to stay at Mutiara, but they were full, so I was shown Laguna - I liked the room and stayed there. No resteraunt, just a simple breakfast and tea/coffee when you want it. I ate at a few different places - Mutaria does a big buffet meal if you book in advance, the Delfika resteraunt had some good dishes, and we also had a very tasty meal with lots of dishes privately organised one evening at an older lady's home (she used to work as a cook somewhere).
I stayed 9 days, and dived 14 times. I can't remember the dive site names as I did'nt keep my log book updated.. But from memory Pulau Karaka (my 'check out' dive, pretty shallow corals and mating cuttlefish), the lava flow dive off Gunang Api (really interesting to see how the lava has been totally covered by new coral growth over the last 20 odd years, lots of very healthy coral (staghorn, lettuce, etc), but relatively low biodiversity and not lots of fish). Then a couple of dives off Pulau Pisang (enjoyable wall / slope dives), a night dive for the mandarins, a trip to Batu Kapal (great pinnicle dive, lots of fish action, great viz when I was there), an exploratory dive off Gunang Api checking out a possible 'muck' site (I enjoyed this - saw a big crocodile flathead, lots of razorfish, various little things), a day trip with 2 dives to Pulau Ai (more wall / slope dives with lots of fish), an early morning daytrip first to Hatta Atoll (hoping for hammerheads at 7am, as these had been seen there by a few different livaboards passing through - we were not so lucky as to see them, but had a fast drift dive and saw huge clouds of red tooth triggerfish, a shark in the deep, big 1m+ tuna, barraccudas, turtles, etc) then after an early lunch on Hatta (check out how close the 30m dropoff is to the beach in my earler dive boat pic!

) we did a dive into the 'hole' off Pulau Hatta - another good one. Then there was a good dive off the NW coast of Banda Besar with a very beautiful swimthrough, then a wall, then a ridge with lots of fish action (napoleons and bumphead parrotfish).
The only island not regularly visited would be Rhun, because of the distance... But if you're willing to pay a fuel surcharge I guess a visit there could be arranged (a fuel surcharge is also paid for visiting Hatta and Ai, split between the divers). I was happy to do 2 dives a day, but Blue Motion seemed willing to be flexable for customer requirements. I could have done a night dive every evening, but I was also just there for a relaxing holiday
At least a week on Banda would be good - some days diving, and some days exploring - climb the volcano, see the plantations, the different islands, beaches and villages, the various forts around the islands, and Bandaniera is a nice little town to explore..
Not sure if the dive shops close. There is another dive shop in Bandaniera, called WaloWalo - but I don't know much about them, and i never visited their dive shop. I met a couple who enjoyed diving with them, and they saw a manta. But I think June - August has more wind and rain (others might know more).
Alor or Banda... Purely for the diving I'd say Alor, because of the mix of muck and reef, warmer and cold water sites there. Banda diving is more great viz, wall diving, and lots of fish. But as an overall holiday destination, the beauty and history of the islands make the Bandas a very special place to visit in my opinion! Hope this helps!