Hi Sebastian. First of all, thanks for reaching out and sorry to hear you have so much trouble with the game; I would totally understand if you would request a refund.
About your Linux problems: there must be something special going on in your case because dozens of Linux players have already completed the game with no problems. One thing I noticed is that the draw distance for body parts is extremely short for the lowest quality setting, also in the editors. This will be fixed in a future patch somewhere in the next few days... but for now could you try a higher quality setting to see if that improves things?
About the frame rate drop with animals in Windows: I discovered today that it is possible to create animals that give problems with the animation system, resulting in the game basically becoming unplayable: it's a combination of two types of feet and very thin animals. Perhaps that's the problem for you too? This will also be fixed in the next few days.