There's one massive, giant problem everyone here arguing 'lore' keeps overlooking; The machine spirit!
Any imperial vessel we board will refuse us access. Unlike every previous encounter our only two tech-adepts have had with the machine spirit, this time, it Will be actively manned. It will fight us.
As for realistic, well... if we're a small enough vessel that a single person can pilot it, then we'd be a smear on the hull of even the smallest imperial craft, which starts with solid adamantium walls something like ten-twenty feet thick. As what we're on isn't a boarding craft, the only thing we could do is ram it, and our own structural integrity was already ruined by the missle that hit us.
Now, from My perspective;
No one knows how many eldar actually crew eldar vessels. All that's known is that eldar are prone to using spirit stones in place of A.I., which end up acting much like computers. Also, we are talking about craft-world eldar. While largely warp based, their technology is one of the most advanced in the galaxy, second only to necrons. Necrons, of course, have massive war ships that are unmanned. An eldar 'skeleton crew' could very easily be a couple hundred bodies. Easily something our group, with some warp back up, could take.
In terms of realism, again; we're flying a ship made of something supposedly capable of nullifying the presence of hundreds, if not thousands, of eldar spirit stones. As far as lore is concerned, what we're flying doesn't exist. But, if it did, wraith bone would be disrupted by it, as it's literally solid warp energy. Therefore, we'd actually have a chance in hell of surviving running into it.
As for being based in lore... Everything I've been talking about thus far is proof of concept. Of course we're not going to make literal death-masques. Just something based off a concept that has been proven to work, i.e., making servitors out of chaos spawn. Given a practically unlimited supply of the most versatile material in the galaxy to make whatever parts we need, and access to centuries-old warp knowledge, it's well within the realm of reason (especially by 40k standards) that we could pull everything I've said off.
Most importantly, though... Why are so many of you so infatuated with keeping everything human? Science fiction is about exploring the strange and the unknown. The vast wonders of the universe of the 41st millennium are ours to take hold of and explore in our own way. To a man, each of us decided, at least originally, to play a non-human entity. Not a single character among us could reasonably be called human, let alone have lived a life that could hope to match. So why not take things a little over the top? It may not be the dark, broody, depressing galaxy of the 41st millennium the human race lives in. But it sure as hell is the anything-goes, chaos-infused 41st millennium of the filthy heretics and dirty xenos!