Hello and welcome to BW forums
I'm sorry to hear you had such a rough first day with the shush/pat method. Sleep training is hard work and Tracy said EASY wasn't easy, but regardless of knowing it is hard work I don't think it's right that you exhaust yourself to the point of shaking or being in tears. Your health and mental well being are more important than sleep training and I am positive everyone in this community would agree.
If going for one nap per day would be more manageable then sure do that, lots of people do. You can expect it to take longer overall but it is also likely to be more manageable.
You can pat on her side/hip/nappy area rather than on her back. You are correct that sleeping on her back is the safest position, you can adapt the patting to a more easily reached are. Of course you wouldn't pat too firmly on LO's tummy but they are usually so well padded by a nappy that the tummy/hip is usually fine. Some people are able to manage shifting LO from their side (patting on back) to their back (for sleep) after LO is asleep but personally I would rather just put LO down on their back and teach them to fall to sleep that way - it's your choice.
Yes you can use W2S (wake to sleep) and begin shush/pat at 35 min and continue all the way through the transition into the next sleep cycle and through to deep sleep (20 min later) but it sounds like you have tried patting throughout the entire nap without getting any additional sleep. I would certainly put a limit on how long you are trying to pat or resettle as it is draining on you - please do take care of yourself. Many people choose to "give up" after 45 min but there are also lots who decide 20 min is their max and if LO isn't back asleep by then just get them up and move on with the day. Others decide not to try nap extensions at all as we know it is hard work and they choose to get LO up as soon as they wake even after 45 min nap.
Any of these options are fine with us here on the forums. We obviously promote and support the BW methods but we are also mums (and dads) ourselves and know full well there is a limit to our sanity threshold.
I hope this helps some.
Please also take into consideration that your LO is still very very young. Respectful sleep training takes time and is not going to happen right away, it is a gradual process. Don't feel guilty for holding or carrying baby for sleeps at times, the bonding is vital and actually helps builds LOs confidence and trust in you which is needed for independent sleep any way... be gentle with yourself.