How big was the Imperial Starfleet?

?

 

Missed this post last time:

The context seems to me that he is not talking about the Death Star's planet-destroying laser. Instead, he is talking about its defensive weaponry. Dodonna is telling the pilots what sort of dangers they would face on their mission.

It could be. I've always taken it to mean the total firepower of the station and only in the second part was he talking about the actual defenses. In any case, further discussion will assume that he was talking about only the defensive lasers, in order to get minimum figures for fleet size.

Rergarding the number of turbolasers on the Death Star:

I searched around on the web and it seems to me that the 12,500 turbolaser stat came from the RPG, so I don't know if I believe the number or not. Besides, if the Death Star was 120 km in diameter, this works out to about 0.28 guns per sq. kilometer or 1 gun every 3.6 sq. kilometers (each gun would then be spaced 1.9 kilometers apart). We know that in important areas (like the trench) there are more guns, so over the surface they would be spaced even further apart. I don't know if that's the case.

Celebration scenes after only a fraction of the fleet was destroyed?

The celebration scenes in the SE don't necessarily mean that the Empire was completely defeated right after Endor. I don't even think they took place on the same day. This is because if the Imperial fleet was small (200 ships or so), less than half the ships were destroyed anyway. The Empire still had stormtroopers on every planet to enforce order (they must have had them). The remaining Imperial government didn't need to surrender right away after the Emperor died and they would not have to allow the people to riot like that. We do see people celebrating with no fear of troops coming in and killing them, which suggests that the celebrations took place after stormtroopers were defeated. It's unlikely that a civilian public was able to do this within a day after the Emperor died.

The only way they did take place right after the Emperor died is if the majority of the fleet was at Endor (it shouldn't be, because if that was most of the fleet, the Rebels would have noticed that the fleet was no longer on regular patrols), there are no stormtroopers on planets enforcing order (unlikely, how does the Emperor maintain civil obedience?), and Imperial governors decided to surrender (if Tarkin was any indication, they wouldn't).

So, if the Empire didn't completely die at Endor, then it mitigates concerns that there could be a celebration when only a fraction of the fleet was destroyed simply because in either case (big fleet or small fleet), the celebration shouldn't have taken place that day.