Anyone feel free to shoot me down on this I normally remember how to do things for only 1/2 hour after I've done them! anyway...

1. to take out factions from a campaign I think you only need to remove them from descr_strat.txt and descr_regions.txt, in descr_strat re-assign any settlements they own to other factions or slave and remove completely the rest of their listing, remove them from playable/non-playable list and remove their diplomatic status lines, not sure but you may also need to remove any slave armies that are sub-factions of those removed. In descr_regions.txt change the default faction (3rd line down) to a faction that remains in descr_strat. To stop slavs emerging you can just delete their trigger from descr_events.txt and they will never appear.

2. give them their province and create a starting family tree for them (suggest copy a relatively short one from another faction and substitute Romano_Brit names). Remove ", spawned_on_event" in sm_factions.
To disable their emergence trigger - which does cause problems if left in - simply remove or rename the 'britain' hidden resource (effects descr_regions and export_descr_buildings)

3. yes I think so, if you remove the shadowed_by and shadowing references in sm_factions and give them 'normal' listing in descr_strat they will work like a normal faction. If you want them to continue taking over roman provinces on rebellion don't change shadowing in sm_factions. (I think the rebellion can be buggy especially if the empire/rebel factions besiege each other but don't know if this is made any worse by having them on at the start)

4. to use a new map, if his works as a provincial campaign you should just need the world/maps/campaign/campaign_folder whether that works with what you have modded so far will depend on if you are both starting from the same version of BI and what's been modded at the higher levels.

Suggest you start by getting the map to work with whatever you have done so far. You'll need to start from his text files in the campaign/campaign_folder so that the descr_region colours tie up with the regions_map.tga and adapt the descr_strat file to suit any changes you may have made to buildings / units / family names etc. Don't forget you also need his campaign_folder_regions_and_settlement_names.txt in your
data/text folder.

Once you get anything working make a back up of it and then try the next step. Good luck.