Are you saying it’s tangled like the bobbin thread or top thread gets tangled when you sew and creates problems or the tails left over from when you cut the thread from the last stitch? If it’s the later you can usually pull the thread back through the stitches and trim them.
For your second question. Time and practice is always the most tried and true answer. But they also make feet with edge guides. You put the guide on the seam and it rides there making the French seam stitch straight and equal distance from the original seam. You would then have to do this the same going back on the other side. This would involve rotating the material in your machine. You can see this foot down a few pics.
http://www.thehogring.com/2017/04/18/ho ... ench-seam/
Another option is to do almost the same thing but find a spot on the walking feet you normally use and keep that mark or spot over the original
Seam to keep the needle the same distance apart. This is more the time and practice area. No method is a guarantee as they can all sew crooked. Best of luck