Actually, weapons roll from 0 to weapon attack value. So for lasers, it's 0 to 1.
To answer your questions:
1. You can't, not really. You can go to war with them to divide their firepower up, and you can shoot down their transports so they can't invade the weaker races, and you can bribe them to make peace with the weaker races, but at some point a race which has military strength is going to war with a weaker race. Unless of course they form an alliance with the weaker race.
2. No idea. It seems though that the AI remembers how badly the war has gone or considers relative military strengths and uses this as a factor when negotiating to end the war. So maybe it plays a part in surrenders as well. Perhaps the way it's designed is supposed to encourage you to intervene earlier and build up a good military of your own.
3. By the time the enemy sends fleets at your planets determined to obliterate the defences, you need to look at building more powerful defenders (remember, you're not limited in terms of logistics here so you could park eight battleships in orbit if you wanted to), slowing down incoming enemy ships with warp disruptors (available on military starbases only) so that you can get a defensive fleet over there to mop up the enemy fleet, and/or striking at the enemy's manufacturing planets so that they can't put together any more super fleets.
You can use espionage to figure out which planets to target - either train a spy and just browse the enemy planets without actually planting him for a sabotage mission, or use the race-wide spy slots to get the intelligence level up to high, that way you can see the planet screen for any of that race's planets.
Don't worry about losing planets. Just make sure that you have some warships and transports stationed at rally points all across your empire, then you can zip over and take your planet back in a turn or three.
If you would actually prefer to pursue a scorched earth strategy, keep some nearly-empty transports around (manually launch them from a planet with just 1M troops on board) and land one on the newly-conquered planet to pick up all of your surviving soldiers, leaving it with a population of zero. Then click the Details button on the planet screen and then the Abandon Colony button. The planet will now be uninhabitable and the enemy will not be able to claim it back, but you can either re-use the surviving soldiers or send them back to one of the planets that you're keeping.
When the enemy is sending fleets to intercept your transports it's important to have more than enough ships covering them, because transports tend to die easily. Quite often I've found that in the late game, I need at least two full-logistics fleets covering one transport fleet, just in case.
When you have to declare war on an empire that has an ally, don't forget that just because the ally declares war on you, doesn't mean you need to fight them for the whole duration of the war. You can sign a peace treaty with the ally without stopping your war with the other empire. This can isolate enemies from that support and gives you a much better chance at wearing them down.
Wars can be costly, especially if you have an influx of new planets with high-maintenance improvements and low population. So if you aren't going to divert funding to Social during war time (I prefer to keep it all in Research and Military) be prepared to sign a peace treaty to avoid economic meltdown.
Don't be afraid to start wars when you see that an enemy is researching hard. For one thing, because he then has to eventually spend more money on building new ships, you're slowing down that research anyway. Also, if you don't get in there before he finishes perfecting that cool new weapon technology, you'll have to face much nastier and tougher ships. Take out those planets which have labs on bonus tiles - they're costing him the least in maintenance, but giving 100, 300 or 700% more research. And that just won't do now will it.