I don't understand anything about MP editing, but:
Not sure how to do it in OFP per se but at least in ArmA1 (the last time I tried to understand the MP side of things) the trick was to run several instances of the game and the server on your own PC, then connect the games via internet to the server and it fools the game thinking it's in actual MP.. Setting this up was I think a bit convoluted/cumbersome (but then again what has BIS ever done that wasn't), but that's how I tested stuff. Can't remember how it was done but at least you need to set the server.cfg not to kick duplicate IDs because otherwise you can't connect to your 'server' with more than one instance of the game..
You can also use the so called 'MP editor' which is some sort of semi-MP (where the game thinks it's in MP or something) so respawns and whatnot work, you go to 'Multiplayer' -> host a session -> select island and you should get to the editor from there. But that doesn't yield same results as testing on a dedicated server AFAIK.