What if they decide never to give you your stuff back or take like 6 more months? When you buy the assets and models, is it impossible to make the same characters again, if you don't have any choice because they won't give the drives back in any reasonable time? It just feels like your suffering too much, and it might be you need to bite the bullet and just redo the characters that where in the mnm game, which will then let you get on with your livelihood. I know it's obviously not idea or anything, but at the very least you can work and have everything you need to make the new chapter.
It just seems like it might be the only way until at least they either give the drives back or not. The police seem to be a law unto themselves, even if your innocent which you are, the bastards will still not give them back for a very long time, which is an utterly ludicrous situation for you. You should give it some serious thought, remaking the characters etc, and once you have done that, your not so dependent on the drives the cops have.
Or you might want to consider getting a different lawyer, one who can get this problem sorted. It's been 3 months already and with no sign of any action whatsoever and your lawyer by the sounds of things, seems very reticent to actually do anything at all. I know the cops in your country if they move any slower they will be like statues. At least if you remake your characters etc, it might help you with your job financially, and also allow you to get people in your patron to beta test the remade characters etc. Just a thought.