I'm beginning to think airsouthwest (who has disappeared and deleted many postings) has re-appeared under a new username...
Any 2 airlines can enter into a codeshare without IOSA accreditation or a safety audit. Most airlines do go through this process, but it’s not mandatory.
If you are in IOSA - which virtually every major airline is - then you can only codeshare with another IOSA airline
or if you undertake a pretty comprehensive safety audit of your potential codeshare partner. If Flybe wants to codeshare with Isles of Scilly Skybus (for example) then neither are IOSA and they're free to do what they want. If Flybe wants to codeshare with KLM which is an IOSA carrier, KLM is bound by the rules of IOSA and has to either ensure Flybe itself is IOSA accredited (which it isn't) or perform a full safety audit. Same goes for almost any other major airline with whom a codeshare would be worthwhile.
It explains why there are no codeshares with Flybe as yet. You can't just go around collecting codeshares like bottle tops.