Yes. It is illegal for you to open anyone else's mailbox for ANY reason. You are not allowed to put anything into someone's mail box, either.
Yes, unless you have permission by the person you are using the name of.
Yes, it is still trespassing on another person's property
If you have their permission, yes. Otherwise, no.
If they were unused checks and you had not signed them, then it is definitely Illegal. Using another persons check without his/her signature and permission is an illegal activity. Even if it is the spouse using your check.
Obviously YES. It's technically stealing. Yes, it is illegal to use another person's cell phone without permission since that is a form of theft; in principal it is illegal even to use another person's coffee cup or pencil without permission, although such offenses are very petty and it is doubtful that the police would be willing to investigate them. If the unauthorized cell phone use resulted in a large expense for the cell phone owner, that is a more significant crime.
No you don't. Simple oral permission will suffice.
Yes
If you view or alter any files on a computer or online without permission, you are breaking the law
As far as the mail is concerned - it is covered by US Statute and it is illegal for anyone to take anyone's mail out of the mailbox for any purpose. In fact it is even illegal to place anything into a mailbox that was not delivered by the US Mail - the Post Offcie can and many times demanded that postage be paid on those items. What do you mean by "hold" their mail? Either you live somewhere or you don't.
YES
No, it is illegal (unless it is done by the parent of a minor child).
My permission.