OK, we can call that zero MG for all intents and purposes then lol - though it is surprising that the TDS is so low for KH 12 + GH 17? (Edit: Just done the maths - 17.8ppm ~ 1gGH, so 120ppm ~ 6.75dGH - I'm not sure what else could contribute to a GH of 15-18? (perhaps
@dw1305 knows?)
In that case you could do as I suggested above and cut with 20-25% tap water and bump the GH up with some Mg.
The first two should be fine with your cut tap water, but the Blood Vomit requires very soft water.
If I were you, and if you have a ready supply of rain water, I'd be tempted to use 5-10% tap water to get your KH down to 1 or slightly below, and then remineralise with MGSO4 and CaCl2 (if you want to use DIY salts) or Salty Shrimp (if you want to use an off the shelf product) to bring the GH back up to around 5. Then your low KH should ensure you can grow almost any softwater plant you want to.