Newbie questions: How do you debug an interpreter when you aren't coding for it directly? How far does fuzzing strings of opcodes get you? How much practical

Newbie questions: How do you debug an interpreter when you aren't coding for it directly? How far does fuzzing strings of opcodes get you? How much practical difference is there between a server side WASM engine and a browser-based one? How much work would be involved converting one to the other?