I'll tell you - you're just being paranoid. None of my children crawled on hands and knees until they were 12 months old. They commando crawled and got up and rocked for MONTHS before they crawled. Two of them walked at 18 months. Sitting up happened at around 8 months, and from memory they couldn't just sit for hours, they would topple after 20 minutes or so.
It sounds like your baby is just fine. I used to mention what my babies were up to in their milestones at doctor's appointments and not once was my doctor concerned. There is an age range for milestones for a reason. So when it says crawling 6-12 months (or whatever it says) that means some babies will crawl at 6 months, some will crawl at 12 months and it is all normal. Please stop stressing. My kids are all school aged now and they are all fine.