EAD allows you to work without restrictions, so both a) and b) are ok. To get your GC approved, the best case is to find a similar LC job at the time of RFE/interview. If you can't find a similar job, being employed is better than not because at least you can prove you are not a public charge. If you read AC21 literally, it says it is okay to have non-similar job as long as you are not a public charge.
Having said that, I never heard of any approved case that people respond with a different job for RFE/interview.
Good luck!
