The Long Las is silenced. How ?

The sound of a las weapon shot is the sound of it heating up the air it passes though. It doesn't matter what one does to the weapon, you'll still be heating up the air, so you'll still get the mini-thundercrack.

Why is the sniper rifle reliable while the long las isn't ?

When the long las is going to be the one with less moving parts.

Why has the clip size of las weapons been so drastically reduced ?

Having a large clip seemed to be a major feature of the las weapons in previous games. But that's gone now. Sure, there are now other reasons to pick the lasgun over the autogun or laspistol over SP pistols, and weapons better than both are now within eaiser reach. But it still feels like las weapons have lost something that makes them distinct to SP weapons.

The Long Las is silenced. How?

You got me. Hand-wavium? Muzzle flash suppression, maybe. Silence? No.

Why is the Sniper Rifle Reliable while the Long Las isn't?

You got me. Hand-wavium? Discharge heat wears down the barrel?

Why has the Clip capacity of Las weapons been so drastically reduced?

You got me. Hand-wavium? 150 years after the Calixis Sector, technology wanes?

All joking aside, being able to silence a Las weapon is probably meant to maintain balance between Las and SP, considering their other differences. Maybe. I couldn't begin to tell you the game dev reasoning behind a Long Las not being Reliable. Or why the Sniper Rifle is. I read somewhere the Clip capacity on a Sniper Rifle is really low. That could be why. As for Clip capacities in general, it may simply boil down to the game devs wanting to ramp up encounter tension by bringing ammo usage closer to the fore- an old Calixis Las Pistol might have had 40 or more shots in it, and that's what, two, three full encounters? Drop the capacity by half on these Askellon babies and a bad fight runs everyone out of ammo. That's all I can think of. It's not like "buying" standard ammo is going to be as much of an issue as it had been. When I was playing I kinda enjoyed scavenging used Las cells- this one had 37 shots remaining, this one had 54, and so on. Never spent a red Scint on Las cells.

The easiest way would probably for it to be a slow continuous burn, or multiple short pulses of lower intensity.

In either case though, the silence would be on the firer's side. There's just no avoiding the little vaporizing explosion at the target point. Could be worse though, like a Bolter. Kaboom