Sheets are fine for the back and many are of better quality than the fabric used for the top! Just as all 100 percent cotton quilting fabric is not the same ( lots of discussion here on that subject) neither are all cotton sheets. Yes a high count sheet will be difficult to hand quilt, and if you make a mistake quilting it there can be holes from the needle. So buying a 200 thread count to 150 thread count cotton sheet is comparable to cotton quilt fabric.
As for the poly thread , it is a myth. enough said on that subject.
As for bearding from a poly batting, it depends on the quality of the scrim. A high quality poly batting can perform well. It is the quilters choice for the desired outcome ,warmth, draping, hand vs machine quilting, many variables come to play in selecting the batting for a quilt.
There are many individual choices and circumstances that are enveloped in the final quilt, each should be a treasure and free of judgement.