I would likely use an off white/cream ish color. Though the gray options that everyone else is suggesting would work as well.

It helps to take your stitch length down to "2," when thread color showing through, might be a problem. Shorter stitches keep the seam closed better.

In my opinion, it doesn't matter much. I make sure my tension is correct, and I use Bottom Line (60 wt) pretty much exclusively. The thread is rarely visible on the top side of the quilt. The only time I make sure I use a certain color is if I'm working with light fabric, I don't use black or dark thread because it can show through the fabric.

Gray for sure.

I try to match the fabrics and threads when ever possible. If I can't I go with a med grey or white. This is a personal choice.

I always use the light color. Or whatever thread happens to be on my sewing machine at that moment, lol.

This depends on what the quilt is intended for. If it is for steady use in my home I tend to use whatever is handy. My thready stash exceeds my fabric stash since I did only finished garments for years. If I intend the quilt as a gift or show then I am very particular.