There is currently no limit on how many Xenons you can add to the MESH network.
Sleep Modes are working on the Gen3 devices now so you can use them.
Ideally your going to need external bluetooth antennas to get better range than just using the built in antennas.
If your going to try to have the Xenons sleep and not use solar then you will not be able to rely on the MESH networking to extend your range and will need to rely on direct communication between the Boron and the Xenons using external antennas for best range.
I think you should be able to do Direct communication between the Boron and the Xenon with antennas as long as the Boron is as close to the woods and as line of sight as possible with good antenna placement.
The only thing that may be an issue is if the MESH reliability. I haven’t tested the reliably with the latest firmware but on the previous versions I would lose Xenon connectivity when the Boron would lose cellular signal and reconnect automatically. That was with Xenons that were powered 24/7, if your Xenons are sleeping and then waking up and reconnecting then this may no be a issue since they attempt a new connection on wake up.
I would try the Boron and Xenon combo before buying a bunch of RF radios and different microcontrollers since if it does work it will be a lot easier to code and work with since you will be working in the Particle ecosystem and will just need to use Mesh.Publish commands which makes things easier than trying to work with XBee radios their more complicated code structure.