I don't know if you have come up with any of the solutions needed for the mechandrites.
But to create a mechandrite like the standard model, you could get the basic design of a desktop lamp with adjustable height.
Like this one.
Take the workings of it and scale it up.
You would need to bear great attention to detail, but in the end you will have at least an adjustable arm that can be mounted on a backpack. Or similar.
The mechandrite itself could be a simple lightweight whatever mounted on the end of the mechandrite arm.
On the bolt pistol problem i know i've seen a few tutorials.
Oh looky, i found a site that have taken ALOT of 40k cosplay things.
If you run out of time on the axe, you could just get a chainsword.
There are power weapons as well. Marketed as vibro sword.
The stuff can be found in other shops as well.
The servo skull i would make with such a hallowen decor, and adding whatnot and bitz that looks good with greenstuff and whatever means practical.
Then paint it. And finish off with a few dull coats of varnish.
Shiny varnish painted on the "metal" parts.
Don't know if i know anything else, i have these problems myself.