On building a coherent variant combat system...

[Celebrim]

it seems that the effect of your overarching house rules (low magic campaign, gritty-er view of combat) puts the PCs at a disadvantage.

At a disadvantage compared to what?

Because the only way I can understand that sentense is that you are trying to say, "Puts the PC's at a disadvantage compared to the PC's facing a similar challenge in a different campaign."

And it seems to me that that is not a valid comparison.

Now, as for your actual suggestions about the house rules, I agree that this first iteration needs alot of reworking and additional thought (which the OP seems to be doing). But whether it puts the 'PCs at a disadvantage' doesn't seem like a valid basis of criticism.