#1
How does building an uncovered humbucker differ from a covered one? You only use slugs, no screws, right? Also, do you use the metal spacer on the bottom or just the two plastic shims?
#2
There are still screws, unless you don't want any of the poles to be adjustable. It's up to you really. It's essentially the same process, minus the cover. You also have to take into account the tonal differences of cover vs. no cover.
My Tele Project!

Cheapy Johnson Steel String Acoustic
Les Paul Ripoff w/ SD SH2 and SH4
Modded Crate V18 2x12 w/ Weber Speakers
Squier Deluxe 5-string Active J-Bass
Acoustic B100 15" Bass Combo Amp
Gray VS Jekyll & Hyde
Vox V847A Wah
#4
If you surround a magnetic pickup with a conductive material like a cover you're going to remove some of the treble content and I think it bumps the upper mids slightly.
My Tele Project!

Cheapy Johnson Steel String Acoustic
Les Paul Ripoff w/ SD SH2 and SH4
Modded Crate V18 2x12 w/ Weber Speakers
Squier Deluxe 5-string Active J-Bass
Acoustic B100 15" Bass Combo Amp
Gray VS Jekyll & Hyde
Vox V847A Wah