Hey there Louis,
Yeah, I'm gonna go with pretty much exactly what Tedtan said:
- lead singer has to be comfortable with it, so his key choice trumps others in the band. I am always reminding myself that that the audience is 99.836% "regular folks" --i.e. not musicians or tone geeks. So when they come to hear "music," they're really listening to the singer ...the rest of the players are just support crew
- transposing up or down certainly changes the chord and single-note voicings, to be sure. This change is fairly straightforward to mitigate with playing elsewhere on the neck, higher or lower, to try and capture (retain?) that timbral flavor from the original key. Whether playing those lead lines in a different position is "enough" for your ears is something you guys all decide during rehearsal ...no substitute for hearing it out, making changes, then deciding to keep/ditch at that juncture.
- the bassist is important here to fill whatever "bottom" you feel may be missing from your lead guy transposing the song. Doubling up or counter-melody lines go far in this respect. I play with a pretty tasty bassist who fills the space well ...we're a trio so sounding fuller is a biggie for us.
- playing lead lines capo'd? No stinkin way I would ever do that! Unless your lead guy does absolutely
no bends, I personally don't think using a capo is a good idea for lead lines for rock covers. But heck, it's easily likely your guy is bajillion times a better player than I am, so accept that with many grains of salt.
- Hope this doesn't sound silly on my part but what I "hear" you saying is maybe in D it won't sound as "full" as in the orig? FWIW, we also cover Layla (Eric's "unplugged" version) and we play that in Dm. That lead line at the end sounds pretty dang good from where I'm a sittin.

Give it a whirl, man! I have full confidence in your guys making it sound good!!!
Edward