Lr uses an internal color space for all work. According to Adobe
Lightroom uses Adobe RGB:
-for previews in the Library, Map, Book, Slideshow, Print, and Web modules
-when printing in Draft mode
-in exported PDF slideshows and uploaded web galleries
-when you send a book to Blurb.com (If you export books as PDF or JPEG from the Book module, however, you can choose sRGB or a different color profile.)
-for photos uploaded to Facebook and other photo-sharing sites using the Publish Services panel
The same article states it uses ProPhoto RGB for all work in the develop module, although it's been documented it's a slightly modified version of ProPhotoRGB.
So the original file may be in Grey Gamma 2.2, but Lr is converting that into it's own working spaces. As far as the ABW driver from Lr, I don't use ABW but my understanding is that process does not use profiles, but is a printer driver conversion.
I'll let others comment on the workflow and the choice of grey gamma, personally I don't use anything special for my B&W conversions, most are just converted inside Lr using camera raw tools and printed just like a color photograph.