Hi again! I wanted to respond to your suggestions. They are interesting ideas, but I don't feel that much of it falls inside the scope of this mod. I would like to give a more detailed answer:
Unlocking the technology after Vulcanus/Cerys: I am not sure what the problem is, but I won't move the technology to a later stage! It is absolutely meant to be available very early on as a green alternative to coal power.
On the reverse heat exchanger: Keep in mind that if Solar Panels are inefficient on a planet, the thermal panels are going to be entirely useless, because they will struggle to overcome the continuous heat loss (a unique feature of my mod) and break the threshold for steam production (it is impossible on Aquilo, Fulgora and even Gleba at default settings). Solar Panels will definitely be the better option. You could still create a "steam battery" by using an electric boiler, and convert it back to heat with a steam engine + an electric heater, although that may not be the aesthetic that you seek...
The hybrid exchanger faces the same and other issues, and it also seems impossible to create due to prototype constraints!
I am not going to create any overhaul mods with new planets, but your idea with an oxygen-deprived, cold planet with plenty of sunlight does appear to make sense: It could be a planet just moderately close to its star (like Nauvis), but one that lacks greenhouse gases and has extremely high albedo (reflectivity) due to a near-complete snow cover. The reverse heat exchanger could be useful here, but I am not going to create it for such a particular use case, for a mod that doesn't exist yet. I think it makes more sense for others to make such an adaptation.
As for the Basic Heat Pipe: I don't think a lower temperature limit is particularly important for balance (there are also potential issues with imposing that). It already has half the transfer rate and a quarter the heat capacity of the base game Heat Pipe, which makes it much less suited as an energy buffer in (especially large) regulated nuclear setups. Anyway, how does it 'break the game mechanics'?? As for the color, I don't want to change it, since I like it as it is, and it fits well with all the connections.
Those are my thoughts.