I don't think you can beat the comfort that cast iron will give you when it is really cold out, like you experience in winter. With forced air heating, because the heat is carried on the air, these systems it will always feel breezy and drafty. With the cast iron radiators you not only get great convection heating but get all that radiation as well. Forced air cannot begin to compete in the comfort or price of hot water. Here is clue; In Europe where energy cost three times as much as it does here in the states you cannot find a home that is not heated with circulating hot water. Also when you get north of North Dakota into Canada where air conditioning in not nearly so much in demand, 90% of homes are heated with a boiler and circulating hot water. In Montana, in the Dakotas, the upper mid-west and the north-east, circulating hot water and steam are use far, far more more that forced air. They do this for two reasons; one is comfort the other is cost.
Those two 50A single breakers need to be handle-tied, or replaced with a 50A 2-pole breaker. You can't have two singles on a 240V load like that.
The empty breaker space at bottom left should be filled with a proper UL-listed thing. They make blank filler plates, but I find them flimsy and expensive. I just use actual breakers, a CH120 is around $5.
(1) reusing the existing (6 AWG aluminum) wires from the 30-amp circuit to service a new heat pump with a dedicated 25-amp replacement double-pole breaker
Yeah, that plan is fine. Use "MAC Block connectors" to splice from the #6 aluminum to whatever other wires (Al or Cu) you'd continue with.
Note that since the #6 aluminum run does not have a neutral, they must be 240V-only heat pumps. Cannot use bare as a neutral.
You may find it challenging to land #6 wire on a 25A breaker.
(2) reusing the wires from the two 50-amp circuit to service a 100-amp subpanel to serve future wiring projects
Waii--what???
OK, you see that dual 30A and you immediately get "This is a 30A 240V circuit".
But then you see those two 50A singles and for some reason you think that isn't the same exact thing. It is the same exact thing, but somebody left off the mandatory handle tie so it looks weird.
So think of it as a 50A 2-pole breaker.
The problem is, the existing #4Al cable has only 3 wires and cannot supply a 120/240V subpanel.
- You need the bare wire for safety ground. This isn't 1963, you need a ground wire.
- Because it's #4 or larger, you can get neutral by re-marking one of the black wires white with tape.
- That leaves you 1 wire left to use for "hot", and that means the far panel can only be 120V.
The good news is, the SE/XHHW cable is allowed 75C temperature, so it can be provisioned to 65A and use a 70A feed breaker. There is no such thing as a 70A/1-pole breaker, so you will need to either re-use a 50A, or obtain a 70A/2-pole and use only one pole of it.
Best Answer
It's more efficient to run electricity a long way and the line sets a short way, so two outside condensers makes more sense for inside units placed far apart.
In general, you can get more efficient units as single-heads rather than dual (or triple or quad) heads (specifically, you'll have to shop and see what you can get, for how much, but that's been the case when I shopped for mine.) More efficient units cost less to run for their whole life, even if they cost a bit more to buy when you buy them.
Your specific unit will have specific limits as to line set length, but a longer line set will contribute to it running less efficiently than the same unit with a shorter lineset.