Hi again Chadd,
Glad that all worked.
The workflow questions you ask are perennial, there isn't one unambiguous answer and as usual - "it depends". I go into a lot of this in my book on SilverFast, so if you wish to explore this topic in more depth I'll be unbashful enough to recommend my book to you. Click on the link under my signature. But let me try to respond here and now to the extent reasonable: My general approach is to get the scan as close to a finished product as the scanning software allows. That way, you bring it into a post-scan image editor in pretty decent shape, and that gives you more degrees of freedom for further adjustments while preserving the basic quality and integrity of the image. So if you find that making luminosity adjustments in SilverFast improves the appearance of texture of the paper in the SilverFast Prescan, by all means do it. It's just important to bear in mind that a scan bakes in whatever you scanned. Also, because you intend to make very big enlargements of these scans, use the scanner's highest optical resolution and largest scan dimensions you can before the Res slider in the image dimensions panel departs from the green zone. Use this panel in "Expert" dialog mode. Also make sure to scan in 48-bit RGB mode.
As for sharpening, I'm a big fan of two sharpening tools that I think are really state of the art - Lightroom capture sharpening in the Detail panel, but if working in Photoshop, the Pixelgenius Plug-in Photokit Sharpener 2. If you intend to sharpen in either of these applications, do not sharpen in SilverFast. Also, either of these tools completely obviates the nuisance of Lab conversions and they work superbly well. While on this subject and going back to your objective of making big enlargements that well show the paper texture, you may find that Lightroom's Clarity tool, as well as its Contrast tool would make a useful contribution. You can play non-destructively with various combinations of Clarity, Contrast and Sharpening to get the effect you like. Also within Photoshop, the Photokit Sharpener 2 package has Creative Sharpeners that will help a lot. The key, however, is to make sure the pixels get scanned-in at high resolution with no clipping.