Is it ok to remove the back cover if it is very cracked?

Yes, unfortunately that's likely to happen if the back glass plate is already cracked but you can minimize the amount of additional cracking in the process. First, heat the back plate well and uniformly. It can handle a fair amount of heat and the more the better for loosening the adhesive. I like to use playing cards to slide under the glass back. They are thinner than something like a guitar pick and cause less of a lift that may further crack the back. Be especially careful in areas that are smashed or cracked and slide the playing card up along any crack lines so that the card is on both sides of the crack. The overall goal is to slide the card around to break the adhesive without lifting or prying substantially in one area. Differential lifting and pulling is what causes cracking.