Well, of course you can, but it might not be as strong as quilting thread and may "let go" in the future. All the work and time you have invested in this quilt--that would be a shame to have threads break. I would go out and get more quilting thread.
Yes, you can use regular thread for quilting. Be sure to use a sturdy thread, because quilting puts a lot of tension on the thread. You will have the best results if you wax each length of thread before you start quilting. To wax the thread, pull it over a lump of beeswax, canning wax, or colorless candle wax.
Yes you can use regular thread. It's not as strong as quilting thread so if you're concerned just go over the stitches twice. It won't make a difference.
Quilting thread is generally stronger and holds up longer during the years of use in a quilt. Using a thinner version is not recommended for years of use. The stronger the thread the more durable a quilt will be in continued years of use. If you must use regular thread then double the thickness.