Arranging for re-inforcements: What Gregorius really wants is a well armed defensive force permanently stationed at the covenant. Unfortunately, on reviewing his options he realises that the barracks and stables aren't built yet, so they don't have room to house such a force even if cavalry made good defensive troops.
So he'll change his goals slightly, and aim for an auxilary offensive force the covenant can use, that's still based and housed (at least for the time being) at the local village. To try to recruit this, he'll therefore wait a day or so after returning so as not to seem desperate, then request a meeting with the village headman. He'll tell the headsman (via interpreter) that they've got some initial leads on the raiders who attacked the previous covenant and, whilst we're not currently ready to strike at them, we are likely to want to in future, possibly with the village's help. To that end, he'd like a band of the villages young men to start training with the covenant, and going on small missions together, so that they can work together better when the time comes. The covenant would of course be prepared to recompense them...
The magical opponents thing is a problem, and one that needs to be overcome. Assuming that he's successful in recruiting the horsemen, Gregorius is going to try to introduce them to magic slowly and try to get them somewhat familiarised with it. He'll start with little things like Constantine, Viola's butterfly familiar (with Viola/Constantine's permission, of course!) and showing them armour which has had Doublet of Impenetrable Silk cast on it (and suggest such protections could be available to them...) and gradually work his way up, although it's probably a forlorn hope that he's ever going to get the horses to be comfortable around Alcimus.
On the wall point, I have drafted a letter to go to the Tremere Tribune (Card & Board Games ARCHIVE & LINKS). It may get modifed before sending depending on people's responses to the Aegis situation.