First, the range thing. The Shuriken Catapult is effective over 18", but a normal guardian couldn't hope to hit anything with more than dumb luck at that range. Avengers, as more skilled soldiers, are actually able to reasonably hit at the extent of the range for the weapon. Look at it like a sniper rifle. A moderately good sniper can hit his target at 500 meters. The greats can hit something with the same weapon at over a kilometer. The weapon doesn't care about the range, but the accuracy is primarily in the hands of the shooter.
And that's without the in-universe explanation that Avenger catapults are of a superior quality. These weapons are used by the Aspect Shrines and not given to Guardians because of the extra maintenance the extended rails and sighting systems require, and as such are only trusted to trained, professional soldiers.
As for the rapid fire thing... A rapid fire weapon is innately inferior to a Assault weapon at the same range. The Eldar weapons, from 3rd to 4th, lost all their Rapid Fire characteristics in exchange for Assault, simply for the reason that the Eldar are supposed to be more advanced.
Second, lasblasters. Back in 2nd edition, Guardians could use them. The Corsairs can use them now. My main issue with Guardians having them again is this. The Lasblaster, like many weapons in the Eldar list, have become (background wise) ritualistically associated with a particular Shrine. By giving it once again to Guardians, you change the nature of the ritualization of the Aspect Shrines, and thus change how the entire Eldar background works. Corsairs can get away with using them because they don't follow such strict guidelines regarding conduct as true Craftworlders do.
I do agree that rapid fire weapons are innately inferior to assault weapons at the same range, and I understand that the Eldar use assault weapons because 1) they are more advanced and 2) the army is all about speed, and who wants to be wasting time not moving, firing and assaulting? I think that point is mitigated slightly by the fact that by their nature, guardians are more stand and shoot, and AREN'T good in the assault phase (well, defender guardians). That said, I'm not too fond of rapid fire rules, I just thought it was the simplest solution. Another option I thought of was:
Shuriken catapult
Assault 1/Heavy 2
Avenger Catapult
Assault 2/Heavy 4
And then you could get rid of the whole bladestorm power for the avengers (I think it's even more fair than current bladestorm rules actually - they wouldn't be able move and fire that turn and they can't assault).
It would be a little... ridiculous though. 40 shots. 26.667 hits. 13.333 wounds. 4.444 kills (math calculated against MEQs). I think it would be fair if the Dire Avengers were boosted up to 14/15 a model though (which would be more in line with other aspect warriors and their statline).
I agree about the lasblaster thing, it would make swooping hawks seem a little... worthless. Perhaps an upgrade for the hawk lasblaster - either S4 or assault 3 (both would be ridiculous). Otherwise, it isn't really right for guardians to get lasblasters, I agree.
On the flipside, how is giving a lasblaster to a guardian diminishing the ritual significance of the gun to the swooping hawks, but giving a shuriken catapult to them isn't diminishing to the catapult's relationship with the avengers? I definitely agree with your point, but I think that as long as there are two different statlines for the aspects to use and the guardians to use it would be fine.
EDIT: I had an idea that I quite like in regards to Fire Dragons, and I'd like to hear other people's thoughts (maybe I'm just crazy). GW seems to not have fully decided what the Fire Dragons' role should be (they're gods at destroying anything in heavy armor, but sort of become a sacrifice, popping one unit/tank and then dying). They tried mitigating this with the Dragon's Breath flamer, but who really take that? Also, I seem to recall some complaints about them using fusion guns, which is exactly the same as the Imperium, and we ARE supposed to have superior weaponry (our arrogance is only matched by our firepower, after all).
So this is what I propose:
Dragongun (name is up for discussion, I just pulled it out my ass because it has to be called something for now). It has two profiles:
12" S8 AP1 Assault 1, Melta
or
Template S4 AP5 Assault 1
The exarch can be armed with an older, more sophisticated, and bigger dragongun called the Dragonlance, with the following profiles:
Firepike with Lance (yes this is essentially a souped-up heat lance)
or
the dragon's breath flamer.
Now, to have both of these options available at once would mean that Fire Dragon's would have to go up to somewhere around 20 points (I think, to be fair), but would anybody really complain? They'd be able to destroy practically anything thrown at them, and wouldn't be absolute sitting ducks when they finally leave their transports. They'd still be vulnerable to most weapons and probably bought in small unit sizes, so I think it would be pretty fair. Thoughts?