Thanks for the comments, as always they are really appreciated.
This is one of my earlier conversion projects. I stayed very close to what was possible to do with the original PrC. Looking at it again with a lot of conversions under my belt, I would spread the abilities a bit more evenly, and not be so stringent about handing out the prerequisites as bonus feats early on. The result would differ more from the original class + prestige class, and hopefully be more playable and fun. Please look forward to a revision in the compilation!
About the shadow companion, shadows are truly nasty monsters with the capacity to be amazing scouts and potentially devastate whole cities in the hands of sneaky players. It would have to be crippled pretty severely to be a summoning option. But it is doable, and since there seems to be a demand, I will make an attempt.