Tell me about owning a House

Has anyone done geothermal energy? Both our furnace and AC unit are extremely old, and when you add up the cost of replacing them plus the modest energy savings plus all the incentives and tax credits, it might actually make economic sense for us to do. I guess the main environmental benefit is really that you are switching to electric and therefore have the future option of more sustainable power sources, although the pure energy savings is not meaningless.

I have an in-law unit that I rent out, and apparently it's not well-connected to central heating (one small corner vent that doesn't do much to heat the space), and so my tenant has to resort to using a space heater, and which, of course, is increasing our electric bill by a fairly large amount. I've looked around and as far as I can tell, space heaters are all going to run at 750W-1500W, so that's not a great solution IMO. Is my only real option getting a contractor to connect the unit to the central furnace?

Anyone dealt with property line issues? I have a very strangely shaped property where parts of what you'd think are my neighbors' yards are actually mine - they are literally separated from my yard by features (a retaining wall and large rocky/sloped area) and not even very easy to access from my yard - you kind of have to climb to them.

I have no immediate use for the area, and the neighbors' kids play in it and my neighbors are great people and I'm 100% fine with the current setup. The problem is I'm concerned that an adverse possession situation could come up down the line, more likely with a subsequent owner than them. Is there some way I could mark it as part of my property or put something on the land, maybe even something their kids could use freely, without having to fence it or put stakes in the ground or something? Maybe I need to talk to a lawyer.