Yes, you can cut bars down, but there are a few things you need to know. First, if it has a slit in the bars, that slit needs to be recreated after cutting the bars shorter. The slit should be in the same spot and the same width and length. Second, if your bars have a starnut inside the bars, such as for ICS, it will need to be pushed further up into the bars equivalent to the amount you cut off.
You can't run HIC with standard diameter bars. It's impossible. They are the same diameter (or close), so one won't fit inside the other. However, IHC is a lot like HIC for standard diameter bars. You'll need a tapered IHC fork to do this.
I've never tried to put aluminum bars on a threaded fork, so I'm not sure how well it would hold. At the very least, you'd need an oversized clamp. I don't think I'd recommend aluminum bars on a threaded setup. You could upgrade to a threadless fork and threadless headset and then use SCS, IHC, or ICS with the aluminum bars, but that's a pretty expensive upgrade.
It sounds like you needed those spacers underneath your headset's dustcap to account for an improperly machined bearing cup. This is fairly common. The top of your headtube should go like this, going up: bearing cup (integrated, hopefully), bearing, headset spacers, dustcap, then bars/compression/clamp.
Envy Reaper bars are aluminum, so the only way to run those are with IHC, ICS, or fullsize SCS. I don't recommend ICS as it is a pain. IHC works great, but you'd need to get a new IHC fork. You could probably use your standard oversized diameter clamp from your current setup. Alternately, you could use the fork you have with aluminum bars, but you'd have to run a fullsize SCS clamp (not a baby SCS).