Mark Zuckerberg did not renounce his US citizenship. He did, however, give up his US citizenship while acquiring citizenship in other countries to avoid double taxation on his assets.
majority
Spain has dual citizenship agreement ONLY with Spanish countries of South America, Portugal and Philippines. Other countries have to renounce their citizenship if they can and then become a Spanish citizen- in your case the US allows you to renounce: so you must.
Yes, but you would need a visa or have citizenship from a VWP participant country.
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).
Not unless you renounce your Canadian citizenship voluntarily.
You have to be outside of the US to renounce your citizenship, so the question of deportation wouldn't arise in the first place.
You never lose US citizenship unless you either renounce it voluntarily or by serving in the army of a country that is in war with the US.
expatriation
No.
Fulfilling the requirements for U.S. citizenship does NOT cancel your Canadian citizenship. Canadian citizenship is permanent, and (unless obtained by fraud) cannot be revoked by the Canadian Government for ANY reason unless you renounce it yourself. Also, the U.S. does not require Canadians to renounce their citizenship when becoming U.S. Citizens.
The is no age limit for citizenship; people born in the US are instantaneously citizens at birth. There was a time in American history when people who were citizens of other countries were asked to renounce their foreign citizenship before being eligible to become American citizens, but that is no longer required. So citizens do not have to renounce anything.