1. Theoretically, PC formatted floppy disks can be reformatted on a 1541. However back in the old days I got a much higher rate of read errors when reusing a floppy disk previously used in a PC than with any other disk, reformatted or not. I don't know if it has anything to do with MFM vs GCR encoding, or that PC floppy drives have stronger magnets than Commodore drives have. Then again, your milage may vary and it doesn't hurt to try.
2. The X-series is a practical way to reference to the X1541, XE1541, XM1541, XA1541 etc in one. They all connect to your PC's parallel port in one end and a 1541 type (1540, 1570, 1571, 1581 and clones work just as well) floppy drive in the other end. You can either build or buy a ready made cable. In modern days, I recommend either XM1541 or XA1541 as they are known to work in Windows, Linux etc and still are pretty cheap designs. On eBay, you can often see X-series adapters, which consists of a little circuit board and connectors for a regular 6-pin DIN IEC cable instead of a length of cable. It should work just as well, but perhaps sometimes the circuit board and an extra cable behind your PC may be a bit difficult to fit, depending on how tightly arranged your desktop is.
Regarding using the back side of floppy disks by flipping them over, so-called "flippies", the theory is that the dust jacket inside will catch any dust particles as the floppy disk spins, in order to not harm the disk or R/W head. When you flip the disk over, it will rotate backwards and possibly release some dust that the jacket previously has collected. I have at least 30-50 disks thoroughly used on both sides without ever noticing it being a problem, but then again your milage may vary.
Some of the earliest floppy disks were only certified for single sided use. Cutting a hole and using the back side may lead to undefined behavior. I have some single sided SKC floppy disks that both sides used to work for several years, then suddenly the back side became unreadable while the front side still worked. The single sided disks also feel a bit more narrow, but since it worked to format and store data there in the first place, it can't be completely uncoated.