Good day Orion242,
I am not an expert on PoE and so my comments could be incorrect, however, I believe that the PoE Power Supply (called the PSE ... Power Sourcing Equipment) must handshake with the load (PD - Powered Device) before the PSE connects full juice to the PD. So... if one connects a PSE to PSE I believe that the system would trap on this.
As for the PD gear... All of the PoE reference designs I looked at have an input diode bridge on the PoE pairs and so the PD device would not be polarity sensitive.
Yup... 24VAC Transformers are very inexpensive and are extremely robust. To date I have only had to replace one transformer that died of "natural causes" in 30 years... and this was just a few weeks ago. Transformers may be old tech and have their own issues, but you would have a hard time finding another active electrical device with comparable robustness and reliability.
The only other thing I can add is that Ethernet cabling has current and voltage limitations because of the internal wiring size and insulation. Consequently, this physical wire limitation will ultimately limit the amount of Power that can be delivered on these cables... so if more power is needed... more Ethernet cables (and PoE ports/switches) would be needed.
Cheers,
Sam