Problem with waiting for the destruction of the Ring is that according to the appendices the last attack on Lorien was before 25th March (I can't remember the exact date and I'm too lazy to look it up now).
But something new (oh dear

) has occured to me. Rather than it being a whole army attacking Lorien that our envoy/scouting party runs across, it could be a large raiding party composed of trolls (for getting across the River), Easterlings, and really tough orcs. They're going in a secret direction (across the River) and their plan is to go around the big battle between the Lorien army and the big orc army (the third and final wave recorded in LotR).
So our job is now to get ahead of the raiding party and hold them off as long as we can until we get re-enforcements. Possible twist: no reenforcments available: hold them off ourselves!!
The purpose of the raiding party??? The first two attempts to take Lorien have failed because the Lorien Elves are sustained by the power of Nenya. Sauron's idea? Send a third army against Lorien, the sole purpose of which is to draw off the border guards in a big battle to hold off the orcs; also, to distract the attention of Galadriel and Celebrimbor from the REAL threat, which is the super-tough band of 'commado' orcs-trolls-easterlings that our Lorien envoy/Mirkwood scouting party finds. The commando mission is a suicide attack on Caras Galathon itself to *gasp* kill Galadriel!!!
This might also explain the slaughtered border patrols found by the envoys at the beginning -- patrols of orcs from Dol Guldur have wiped out the border defenses in a very narrow stretch where the commando force will invade Lothlorien in a few days. When these orcs stumble on the envoy, they are afraid that their plan has been discovered so they take the envoy prisoner to return them to Dol Guldur where they can be questioned.
This will allow our Elves to follow and interact with the orcs/easterlings/trolls throughout the adventure -- which will allow all the groups in play to be characters we write (what about the trolls? anyone ever played a troll?)