1) You can use size 270 thread in a standard industrial sewing machine like a Singer 111W if you use a size 24 or 25 needle. I have sewed with size 270 thread successfully with my Consew 226R. The problem is that there will not be much thread on your bobbins because of the thickness of the thread, and you'll be changing bobbins every two minutes. I had better luck sewing with 270 on my Consew 255RB-3 because it uses a very large bobbin.
2) Not all double needle machines are chain stitch machines, I have a double needle machine that uses lock stitches. Additionally, the double needle machines used to sew French seams are not flat bed machines, they are post bed machines like a shoemaker would use to get around tight areas. A lot of them also feed a piece of twill tape onto the underside of the fabric being stitched to make the seam much stronger. A chain stitch is just as strong as a lock stitch. The reason for using chain stitch machines like the Union Special is because the "bobbin" is a full spool of thread. That way you never have to change the "bobbin" thread until the spool runs out. The drawback of a chain stitch machine is that you can't back tack.
)3 No, you won't notice it unless you get within a few inches of the stitching, the human eye doesn't work that way . Your concern is not founded in fact. If you make a french seam with a double needle machine on a curved area, the two stitch lines won't match up between the inside part of the curve and the outside part of the curve. You can make a really nice French seam making two passes, you just have to take your time and keep a part of the foot aimed at the same thing as you sew all your stitches.
4) You can sew Alcantra, (or real leather, or anything else) with a regular pointed needle. The key is using the smallest needle for the thread you are using. The thread will fill up the needle holes. I use a size 22 standard pointed needle for almost everything I sew.
No one lives forever, the trick is creating something that will.
Last edited by DanTwoLakes; 01-06-2013 at 04:10 PM.