For Clovis and Charles : Charles was in World Of C, where the "collective unconsiousness" was, so that's why he could talk to him.
For Marianne and C.C. : It's the particular geass of Marianne that allow that.
For C.C. and V.V. : 2 immortals.
For Lelouch being alive in end ep 25:
-He has Charles code.
Why not C.C. ? Because he has also his geass. (c.c. code = no geass, just immortality)
-The title:
Code geass: Lelouch
Code geass: R2
code = immortality
geass= the geass
r2= R.R. (Rerouch ramperouge= lelouch lamperouge) ( like v2/v.v. c2/c.c. )
So lelouch has the code + the geass... "code geass: r2 lelouch [...]"
^^
- We don't see the driver. If it was a nameless driver, no prob at showing his face.
And Lelouch must hide his...
-C.C. talks to him, we knows, so no point in adding at the end "ne lelouch?"
- Nunally don't have a geass. See Ep 23 C.C. at 4:20
-Nunally saw some flashes when touching Lelouch. Only touching an immortal could do such a thing.
Conclusion: Lelouch is alive, is immortal and has many children with C.C. , gives geass to all of them ( since he has a code so he could) and live happely till the end of time.
^^ :P