It might be possible to power the Arduino off the output pin. Since they are dirt cheap I would just test it.
But like the others I would suggest to use the interrupts of pin 2 and 3.
For my battery powered 3.3v pro mini I use 3.3v pir sensors without the voltage regulator. The standard 5v pir needs at least 4.5v just to step it down internally to 3.3V. On these I had problems with voltage stability because I stepped it up to 5V. Also the battery lifetime was poor.
With the 3.3v pir version this thing runs for months. With the solar panel on top maybe forever.
Thanks again, learned a lot... I decide to give a try there, I also had a look on some components' datasheet and seems to be what I need... Let's see, if end up getting the boards I'll publish a review here later.