Haven't read all the responses but just wanted to say that tummy time is over rated in my honest opinion. Moo HATED tummy time, he would scream and cry so I didn't do it.
He was happy on his back so he spent a lot of time on the floor, just not on his tummy. Once he was able to roll he didn't seem to be so bothered by being on his tummy.
He never commando crawled - started 'properly' crawling at 6 months I think. Maybe 7 months I can't quite remember!