Yes, you CAN activate factions that have been originally defined as non-playable by CA.

Yes, it IS possible (in fact it's relatively strightforward - there are many threads in this forum outlining the considerations for activating 'non-playable factions'.

I don't understand what you mean by 'an online campaign'! Two considerations here -

1) you can mod/amend/change MTW to your hearts content and also play online providing, and only providing, ALL OTHER ONLINE PLAYERS WHO JOIN THE GAME HAVE EXACTLY THE SAME MODIFIED FILES AS THE HOST

2) if you mean an on-line campaign as per the fearfullways clan are currently hosting the any modified files pertaining to campaign play are totally irrelevant - but the same considerations as 1) apply for any participents in the campaign that are playing an on-line battle in order to determine 'ownership' of a province.

If you are planning on hosting an online 'simulated campaign' then I would suggest the following -

a) ensure the host provides ALL files that have been modded (for the purposes of the campaign) as downloadable files.
b) ensure ALL participants in the campaign have downloaed these files so as to ensure ALL players copies of MTW are in-sync.
c) recommend ALL participants to create a 2nd image of MTW (incorporating the new/amended files for adding new factions or whatever) for the specific purpose of plating the HOST's campaign. Obviously, if participants add any amendments to their original MTW copies this would prevent them from playing other games online with other opponents.