You're making an issue for yourself, something that works for hundreds of thousands of us, usng pfSEnse, doesn't work for you.
As the mentionned "gateway-on-lan issue", totally not existent, even when looking it up in Google, but suddenly present on your system, this is pretty much the same thing.
Static DHCP leases are setup in pfSense. Handle them, like creating, modify, delete them in the GUI, and all is well.
There is no need to wipe some hiding database (btw : Google knows where this 'database' is, as soon as you use the correct word: config.xml).
For some reason, most of us do not use a power-backup supply (UPS) for our pfSenbse boxes, and still things work very well. And yes, you are not the only one with power failures.
So : the good news is: there is no problem.
And if there is one : you made it up yourself, so : remove it, and you're ok ;)
edit: using a static IP on the client (PC) which fells out of the network mask of pfSense will break the connection, that is normal.