1 I can say:
a Is this thing a pen? - Yes, you can. (You would say this if you are surprised about how it looks.)
but I can't say:
b Are these people my friends? (This sounds sarcastic, normally you won't say this often.)
c Are those things my pens? (It feels like someone took your pens without permission.)
d Are these people mine? (Very rare, only leaders would say something like this.)
e Are those things mine? (It feels like someone took these things from you.)
f Are those things some pens? (Unnatural, nobody says this.)
g Are those things pens? (Wondering what those things are.)
, can I?
2. I should say "These are some pens" more than "These are pens", shouldn't I? - You can use both of them, they're pretty much the same. The first one is only a little more casual.
3. Can I say:
- The man here/up there/ over there is my friend. - Yes, you can.
- The doctors here/up there/ over there are my friends. - Yes, you can. Although you won't say 'up there' often.