Do prayers to emperor/ Omnissiah actually work?
Yes/maybe/no. Due to the existence of the collective warp effect, believing in a being's intercession is indistinguishable from true outside intercession. The frequency of getting a response is low, but significant.
What happens with soul of servitorized person?
It remains associated with their body. 40k soul dynamics are not quite like the classical Western ones, as long as you're still alive your soul inhabits you.
How cult mechanicus obtains new members?
Recruitment, typically from the lay populations of their Forge Worlds. I suppose it isn't impossible for two machine cultists to breed, but they generally look down on such base activities, a lot of them are chemically sterilized, and eventually their organic bodies will be so far replaced as to make them infertile anyway.
Does imperial guard have any tank aces, or grimdarkness does not allow leman russ crew to live more then 5 minutes on battlefield?
The Imperial Guard is the greatest fighting force in all the galaxy.
Does tau stuff have any machine spirits?
No, but they do have full AI.
What the fuck is agressive sentient lines of code?
Scrapcode. Essentially a computer virus imbued with Chaos, which is about as nasty as it sounds.
Ld4chAn tells about pissed off enough mechanicus setting space on fire with firewall to make it impassable for tau ships.What?
Dem nebulas, man.
Do lasguns work against orks?
Not very well. Orks are hardy and have a tendency to live through organ destruction for a while. You're better off with las than a stubber, but you'd probably want bolters to not have half-dead Orks trying to rip your face off.
What are imperial infantry tactics?
It varies between regiments, but is most often some permutation of human wave tactics.