Suggestion

I think it'd be great for compatibility with other mods if the burning chambers could be replaced with other heat-generating blocks such as the crude burner from Create Crafts & Additions.

Reason

Extra compatibility with other mods and the ability to add and remove heat-source blocks using datapacks. This will give steam power more alternatives in creating steam and steam power.
The generic "lit" blockstate would apply to most furnace-like blocks to prevent infinite heat sources.

Exsiting Mods

Create has the crude burner, although it is intended for use with the furnace engine, I think it wouldn't be out of place to also heat boilers.

I think this would be a good solution to #15 as it would be easy to implement and be very generic with other mods. With the ability to add compatibility through a single tag.