Yes and no - a non-psyker is at zero risk of posession or triggering an incursion. A psyker, even a latent one, is at risk, but this only becomes a real likelyhood when they start using their powers.I always kind of got the impression that anyone with psyker potential needed some kind of outstanding self-discipline (like the Eldar, or soul-bound astropaths,) to avoid daemon infestation. But, well, if you have Orks actually building technology because they genuinely understand it, then they don't need to all be psykers.
It's somewhat like if you have a tap in your sink. It might spontaneously come off and spray water everywhere, but it's more likely to do that if you keep using it a lot and have it under greater pressure than it was designed to withstand.
Eldar are only protected from the warp as they channel their powers through their wraithbone runes, which act as filters/buffers, otherwise they are vulnerable like everyone else (perhaps moreso). They are generally stronger so can resist better, but the vulnerability is there.
There's nothing psychic about a mekboyz understanding, it's implied to be more genetic engineering from the old ones than anything psychic.
I don't see why - Orks are masters at teleportation and force field manipulation, true, but that doesn't mean they'll understand how to build plasma weaponry.Though, in that case, it's hard to see how they wouldn't rapidly outstrip the imperium in the tech arms race. Hmm.