Is it normal for babies to be crawling backward?

Yes. Nothing wrong with a baby who crawls backwards. Remember that each baby's neuromuscular system may develop on somewhat different tracks and still be normal.
Yes. It's perfectly normal. It's just another movement skill. Some babies start crawling backward and then change to forward, and some do it the other way. It's usually easier to push than pull, and if the baby uses their arms more, they go back. If they use their legs more, they go forward.
Yes. Most babies do the squid thing for some time before crawling forward. Some babies never crawl forward, that is okay too.
Yes. Some babies crawl backwards. Most babies sit at 6 months, and crawl a bit later, at about 8-9 months. Then they stand up around 10-12 months. At each regular check-up, the doctor can follow the baby's progress to be sure progress is within the normal timeframes. The doctor can examine the baby to see if there is any unusual reason for crawling backwards. There usually is not a problem.
Yes. This often happens in the course of development when the babies are testing themselves and gaining movement. My own son was a backwards speed demon!