Reusing old service wires from electric furnace to mini-split
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 &quot;MAC Block connectors&quot; 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 &quot;This is a 30A 240V circuit&quot;.
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 &quot;hot&quot;, 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.