Re. 1: it really depends how long you need to setup()… If you want to drive e.g. particle sensors which typically needs to warm up, the problem won’t be the sleep time but the overall power usage when awake. The few mA in sleep won’t weight a lot against the awake power. Also do you want to be awake once a day or every minute? that will make a big difference as well.
I typically use a very basic spreadsheet where I enter my awake/sleep power usage and duty cycle to get an idea on the theoretical lifetime…
But most sensors don’t require a long setup time.
Re. 2: absolutely it is not a hack it is a feature of the TPL5110, when you pull M_DRV to VDD it will wake-up your device.
I think I already posted it, but I don’t see it immediately, I have published a proof of concept implementation which can help you to start. I have nodes running now for months on a 700mAh LiFePO4 battery.
Now, in my opinion this is only worth if you already have MKR nodes. If you don’t already have these consider something else which is designed with low-power in mind.