I would agree that I'd be gutted if they use the Mandiblasters just as an execution animation.

Stealth should be based on a variety of factors, such as: Still, Crouch, Darkness, Cover, which should each individually provide you with partial Stealth(I.E. 45%). Being Still and Crouched in Darkness would give you 100% Stealth.
Scorpions should have an Active Stealth ability they can use to gain 50% Stealth, while the Warlock Conceal would provide 50% Stealth to the Warlock and any nearby Eldar.

Mandiblasters should be a short range(I.E. no damage beyond 8-meters) attack with a cooldown(I.E. 0.5-sec) that is activated through various means(I.E. Charge Attack, D.Bash, Block that should replace clang).

I agree on the Chainswords. DoW2 did great in my opinion as the Eldar Chainswords sounded more like a drill quietly whirring rather than a Chainsaw revving.

The major problem with an Active Skill is that the console players do not have a lot of button, therefore they can not make a whole lot of "Active Items", as that would require to many buttons.
Therefore it is highly unlikely that they will ever make an Active Skill, especially considering that they already have the Mandiblaster to fix.

It only becomes a problem if you make a "whole lot" of Active items or don't map the controller intelligently such as by having Actives attributed to a button(I.E. Left Bumper) and then you press X/Y/B/A to activate the associated skill.
Abilities like Stealth could either be toggled or channeled, with the channel remaining even if the player releases LB so long as they continue to hold the button used to activate the skill.

Mandiblasters should be a semi-active ability, automatically used when certain actions are performed(I.E. Charge Attack, D.Bash, Block).

Hmm, they could just fix that with a Souls-esque bar for selecting what you want to have as your active ability.

I guess they could, as long as it works well for console players, then it should be alright.

Just finished a "game" (more like a slaughter) on Harkus; even when I was staying immobile, crouching (and totally invisible on my screen), Chaos marines seemed to be able to see me clear as day from the other side of the map.

I'm not impressed, to say the least.

You are making it sound as if CSM have a special detection wargear available...

From my experience this only seems to happen with scoped weapons. When you zoom in, the red triangle over their head shows up from far away.

Good to know. Still makes stealth pretty useless on open maps, but I guess that will make Eldar better at defence, if Scorpions can just stay hidden without needing to expose themselves.