"Signing statements" are used to declare interpretation of statutes. I doubt he reads it himself - budget bills are dense legalese. Staffers read it and provide summary for the president. Going by how the politicians describe the laws, I am certain that it is not uncommon for a staffer to compose a misleading summary. I expect Trump does a bit of "spot checking" and demanding explanation.
The sheer volume of legislation and regulation is such that it is literally impossible to read all of it - probably literally impossible to read 5% of it, if all you did was read full time.
Probably right. If I were him I would task each staffer with so many pages and then demand a report on the pages they read.