A- Dig long, dig hard. There is titanium down there somewhere... Also on the surface, you can find it occasionally in the wasteland.
B- Enemies should always be spawning infinitely unless you put something down like a bedroll or I think the claim block stops zombie spawns. It might take a day or 2 or 4 or whatever for them to respawn in the places you looted. There needs to be some sort of timer, I just don't know exactly or if there is some give and take.
C- dunno what you mean. Solar cells were only bought from trader in alpha 16, I don't think that has changed. Look in trade "secret stash".