Will all this be in the game or will this game will feature purely hit scan weapon?

I played Loadout for some time and the problem I see with the game is that hit scan weapon is so much more superior compared to weapons that has projectile travel speed. Not only they hit the target instantly, hit scan weapon also takes less skill and much more accurate than anything like pulse weapon that has projectile travel speed and that advantage increases as the range increase because the players don't have to compensate for the delay between shots fired and hit and the character movements. So in my opinion this game should make all weapons have a projectile travel speed so that everyone is working with the same mechanic.

Also will this game have any system that limits the effective range of weapons especially on snipers and lascannon? Again, with Loadout, the beam weapon is just insane especially when made into a sniper weapon due to its low recoil, insane range and damage, also the fact that it is hit scan makes it even worse. The weapon is so overpowered with it having almost no downside as it is equally good at all range. The result, players abuses the weapon to hell sniping the opposing team members at the entrance of the base right from their the safety of their base. Unless there are player with the same weapon on the opposing team, they have little to no chance of countering it.That should not happen at all in any game.

Some weapons will be hitscan, some will have projectiles. This is down to performance reasons.

A related thing I'm wondering about is weapons how weapons with huge spread will handle hit detection.
Some PvP games I've played only detected hits when the crosshair was on top of the enemy, even if the weapon's spread went outside of that crosshair and visually clipped enemies. I imagine this might be crucial to Heavy Bolter, Storm Bolter and right about any Ork weaponry.

Preferably as long as possible, short range only ranged combat is a joke.

not in a game thats 50/50 range/melee. plenty of 40k ranged weapons are only short-moderate effective range.

Fair question and I've seen some aspects of it discussed quite a bit, lemme see what I remember (please correct me if I'm wrong this stuff is just on top of my head). Some stuff on damage fallof is just plain speculation.

In case of the las-cannon (which would be your hit-scan snipe weapon) I heard that ideas about a charge-up time were floating around. Having to keep your target in your sights makes things considerably harder.
I imagine that there is some sort of max range even if it's just to avoid technical issues. The charge-up mechanic also pushes the las-cannon more into the desired AT role as tracking larger targets is way easier.

As for the rest of the SM/CSM sniper armory it's all ballistic. Damage fall-off on bolters could be very interesting indeed though. Just from my gut I'd say that standard bolts would lose armor piercing capabilities (due to their lower impact speed) but retain effectiveness against unarmored targets since the payload is still intact and piercing flesh is not that hard.

Of more concern are the eldar ranger long rifles. I think they're beam weapons as well. No clue how they're gonna be handled. A charge-up mechanic would be pretty much breaking (except it's something akin to TF2 sniper)

Do the orks even HAVE "snipers"?...

sorry for the wall of text.
TL;DR: SM/CSM Lascannon probably chargeup, rest ballistic. Orks don't snipe. Eldar long las, no clue.

If we're going by short-moderate effective range in 40k, Bolters for example are effective out to around 500m.

and you are suggesting you can clearly see a target at 500m without a telescope? and that the terrain will have open fields of 500+m with no cover or obstructions or fog/dust?

with the size of battles they want to implement I should expect open areas of 50-100m at most before you cant see further decently without a powerfull scope, and even then lots of cover objects to use to close gaps.

If they make the lascannon like the Spartan laser in Halo, we should be fine. The Spartan laser is a bitch when it comes down to shooting fast moving target.