Hi Steve,
An at-best-partial answer to your query about the Hebie 350R-S15 x 42 'glider:
First, I don't have experience with both splined and unsplined sprockets: My current sprocket, a 17T item, is the old unsplined variant, still has ~5 - 6,000 km of wear remaining after being flipped, and then will be replaced with a brand new version. I use a 350R-S15 x 38 'glider, which I bought in 2014. The raised lettering on the back side of the frontmost part of the rear section (!!) says that it can take a 15 - 18T rear sprocket. Maybe the current rear section (15-17T) is fractionally smaller on the vertical plane?
Second, on the matter of splined sprockets and their fit on a 'glider: The issue, as I recall it, was that the first iteration of the splined carrier moved the chain outwards far enough to upset Rohloff's 54 mm chainline. The "slimline" version came later, and that meant a much smaller change in the chainline, no more than a mm or so. The former would have meant problems in fitting a 'glider; the latter, much less so.
Reading Hebie's "Info" PDF and their "Assembly Instructions", I'd assume that they apply to both splined (slimline) and unsplined sprockets. BUT, it may be worth a phone call or an email.
Third, even with the old unsplined sprocket or the slimline splined carrier, you may have some clearance problems between the forward section of the of the rear part of the 'glider, and the chainstay on your bike:
Have attached below a photo of the close fit betw the rear section of my 'glider and the chainstay on my Raven. You'll see that I have trimmed off the outer "shoulder" of the bump-up on the frontmost part of the rear section. That plastic surgery gives about 5 mm of clearance between 'glider and chainstay. That in turn ensures that my 'glider keeps its proper amount of fore-and-aft and up-and-down slop. My Raven measures 440 mm between BB axle and rear axle -- if your bike has longer chainstays, its seat stays may be far enough back that shoulder surgery is not required.
A good-quality exacto knife or something similar will do the trick, if needed.
Hope that's helpful, and good luck.