While it was a long time ago I've driven 212 from Belle Fourche to I-90 and it was OK. Some parts were scenic but there really isn't a lot around there in terms of towns and such. 112 down to Hulett from Alzada looks interesting but if my map is correct it is probably narrow and winding and not suitable for higher speeds...so the 31 miles might be longer, time-wise, than it should otherwise seem to be.
It would be a pretty long day to try to see Custer battlefield and Devils Tower on the same day. If attempting plan to leave Sheridan very early. Personally the drive from Buffalo to Moorcroft is pretty boring anyway so you would not be missing much by taking the longer northern route.
Whether re-connecting to I-90 via 24 or via US 14 and Sundance be sure to allow just a little time (20-30 minutes?) to exit at mile 199 on I-90 (or turn left on frontage road from 111 just prior to the freeway) and go about a mile east to the Vore Buffalo Jump (not sure if the teepee structure is still there). Very interesting and free (donations encouraged). It's where the Indians drove bisson (buffalo) over the cliff into a pit. http://www.s201264329.onlinehome.us/