clamshell issues
Yes, I bought the LFM clamshell adapter (I will say that the clamshell you linked to looks much better.) I ordered the "coaster brake" size, and it did not fit my hub (hub was too large in diameter). I called LFM, and they weren't very anxious to refund my money. They suggested I find a machinist to resize the inside diameter. I used a drill press and a large drill bit, and clamped the adapter together and did it myself.
The next issue I has was that the coaster brake arm requires a standoff to clear the sprocket, and the standoff they include with the kit was for a straight-slot connection to the coaster brake arm, whereas my coaster brake arm had a star-shaped interface. I solved this issue by obtaining a slot-type coaster brake arm, and welding their standoff to my star-shaped cone. Initially I just tacked it, and the weld broke loose and I snapped the axle. I put in a new axle, and this time welded the **** out of the standoff, and it hasn't come apart since. However, the latest issue was caused by the coaster brake arm slipping off their slotted standoff during an emergency braking situation. One solution I was considering is to simply weld the brake arm directly to the standoff.
The next issue I had with the adapter is that the threads in the aluminum clamshell weren't great (metal shavings weren't cleaned out, and threads were a little rough), and the supplied bolts didn't fully engage all the way through the adapter when everything was bolted up. After 50 miles or so, I had loosening/pullout issues (note: I did use locktite), resulting in some of the bolt holes being stripped. I drilled all the holes oversize, tapped them, and installed longer bolts and haven't had any further problems with the bolts.
At this point, I never want to have to mess with it again (or for a very long time), and think a lace-in solution is probably the best way to go. On my bike, the rear brake is definitely a weak-link, and even if I replace the axle and weld the brake arm to the standoff, I have a feeling it will somehow break again. I like that the HD Axle kit comes with a strap brake, and am assuming that it will be better than a coaster brake. What were your thread issues with the Grubee hub? I read a thread about the non-iso threads on the pedal sprocket side, but I think it can be solved by using an electric scooter sprocket. Did you buy your hub from bicycle-engines? Did you use the brake that came with it, and if so, does it provide decent stopping power? How much should I expect to have to spread my frame to accommodate the axle?