Ok, just a couple more things.
If you are unsure how long to keep patting for, I would do a little experiment. Get a little seat - I used a storage box with a pillow at cot height, so that my back didn't ache when patting, and put a clock or watch that you can see ok. Once Oscar is asleep, stop the patting and really observe him - his breathing, any movements etc and see what happens. Watch to see what the time is when he begins to stir. You may notice his breathing change , he may squeek, or do that juddery breath thing. Babies sometimes do this when they are passing into a deeper state. This is the bit that you have to try and get them through using patting, shhshing. Do this several times and it will give you an idea of what is happening. Or you may find that you don't actually need to continue patting for the full 20 mins. With Jacob, I could stop patting, but at 12 and 20 mins, I would have to start again. I would pre- empt it and start patting just before I thought he was going to stir. I had to do it lots of times but it did work.
I also did what One woman did - when Jacob cried, I'd pick him up, p/sh until he was calm, not crying his eyes may have closed but he was not asleep. I'd pop him in the cot and if he started again, repeat. This way you can comfort Oscar until he stops crying.
Yes I think p/sh can become a prop. The difference is though, with this kind of prop you can grade it - ie gradually reduce the length of time, and you can also do it while they are in the cot, which is different from rocking, jigging or using something like a wind up toy - like momma bear.
Hope this helps a bit.
I am not an expert, so I won't be offended if anyone corrects me!
Good Luck
Cheryl