Dual Citizenship (or dual nationality) is allowed in the UK. Many countries don't accept dual citizenship, so you would have to make specific inquiries.
There IS no proof of citizenship for illegal [citizens] aliens. I should say, there is no legal proof of citizenship for illegal aliens.
Yes, it is.
Double citizenship in a federation means that the person has a citizenship of the centre as well as of an other state
Dual citizenship means full citizenship of one country and partial citizenship of the other country whereas double citizenship means full citizenship of both the countries.
Nationality, or citizenship as in dual citizenship [double nationalite, or double citoyennete].
no
Only if the other country does not ask you to renounce your US citizenship (e.g. Norway, Denmark, Spain and Luxembourg require you to renounce your US citizenship if you want to obtain their citizenship).
Yes. It is illegal to marry ANYONE for citizenship purposes only. That person could be deported and barred from life from this country and the citizen that married him/her could face hefty fines or prison time.
No
He is Mexican and American (has dual citizenship).
You can't.
If arrested, you will be deported.