Fair enough - The swap hp for damage would be interesting to see done. In the group I was in it might make battles shorter. OTOH, some of us weren't great at decision making and might have to spend more time deciding how much hp to spend.
It'd certainly make for an interesting puzzle - presumably your defenders would almost always spend a lot of HP because they have so many and healing therefor gives them more.
OTOH, when your AOE types go off, they can hit multiple foes with their spend of limited HP, so maybe they get tempted too
Imho, if you are not great at decision making you just have to spend as much hp as you can each round (e.g. a lvl 5 character spends 5 hp each round for 5 extra damage or 5 extra attack). That way you have the highest possible effectiveness (for a hp cost of course) and make the combat the fastest possible. Of course, some cases, like attacks of opportunity, can be excluded.
About hitting multiple foes, my rule allows to trade hp for damaging only a single foe each time. For example, if you wish to deal 2 extra damage to 4 out of 5 different foes you have managed to hit, you have to spend 8 hp.
Post if you wish to ask about Ruins & Raiders. I do not answer to PMs anymore.