[JB80 (Jarrod)]

The Star Trails software definitely works better using jpegs, assuming you are using startrails.de that is. I'm not certain how much difference you would see to be honest, it might just give you a bit more in processing later?
Still there is no harm in trying both to compare, most people just use jpegs though.

Can you post a pic to show what you mean about the trees?
It is normal for them to slightly blur, I suppose there are a few ways to deal with it like you said you could add a single frame after or you could take a single frame at the start with the trees lit up and add that to the stack.

And yep I would say that the dotted line is from the pause between shots and is another good reason not to use RAWs with startrails as the camera takes time to process each image and that time is long enough for the gaps to be created. It could be down to your memory card and the time it takes to write the image to it using RAWs.
Also it could be that you have some settings like "Auto noise reduction" still turned on in your camera, all these settings should be off as again it takes time for the image to process.
Normally I just set my camera to jpegs and leave the remote shutter on continuous and go do something else.

Once you have it sorted then you can go back and try with RAWs but to get the hang of it it is easier at least to use jpegs.