1) I'm quite sure Steam will offer the whole Steam-Package with BRINK. The Friends-System allows to use a lobby-system on the PC as well, which makes it easier to have a working "drop-in, drop-out" system for Campaign mode.
2) You could probably play with more players than the standard amount. But bear in mind, that this would probably also lead to the same amount of fun as playing Call of Duty with 18 people on maps which are designed for 12 and less players.
3) They have said there won't be a SDK for release. They also said that both Bethesda and Splash Damage are well known for supporting modding.
4)
British, Irish, Jamaican, American are confirmed, afaik.
And I guess there will also be German or French and a Russian one.
In total there will be 8 voices, if I'm not mistaken.