I might be wrong but I dont think Siding master is used for anything but vinyl. From your picture it looks like once you tear off the old clap board, you'll need to strip the entire wall to plane out. Once you get a rythym down, it's not that big of a deal to tap-con the block.
I have no idea if you can install Hardi over fir strips only.
I have no idea if you can install Hardi over fir strips only.