Well, that's... A lotta interest. Probably enough to move to an OoC, so I'll handle that soon.

[@Lugubrious]: I'll try to answer as best I can, so let's split the answer up into two parts.

What the PC's situation will be: this is mainly in regards to their on-summon summon situation. The point in time they are pulled from (relative to their own world) isn't relevant so much as it is a reference point for their abilities at the time. The OP for the RP (as is planned) will jump off from everyone being summoned to the church as per the end of the prologue section of the RP. Further details likely will be expounded on a character-by-character basis.

What they'll be doing: a variety of things (duh), but the primary goals insofar as the main plot is concerned will be to retake regions back from the oncoming enemy forces sweeping the land. This in and of itself has multiple facets, as the enemy forces are not a monolith in terms of ideology or demographic. This thus enables multiple routes to actually REACH the end goal.
That being said, taking the head of the one at the top won't actually lead to much. This isn't an army that'll just dissolve into chaos and disorder if the commander kicks the bucket.