Can an instruction set do everything?
Once a programming language is 'Turing Complete', it can be used to emulate any other language. The only difference is then speed.

The instruction set of all MCUs is Turing Complete. It couldn't really be otherwise, they would not work as MCUs if they were not.

The provision of more instructions above the minimum needed for Turing Completeness makes the compiler easier, and the program faster. For instance you could do everything with a load/store to a single accumulator. However, being able to do arithmetic between several registers allows faster code.

According to this wikipedia page, a properly chosen single instruction can give you a Turing Complete machine. However, trying to program such a machine is really an exercise in what's theoretically possible, rather than having a useful MCU.