Hey Munchkin,
As you might have seen I took interest in a sidenotion to your challenge and startet another thread on this here.
For now it seems that his spells is doable and if so it is a much easier way for you to do your Alarm - and aven at no vis cost. And actually at as low a level that you might be able to cast it spontaneos without spending time in the lab making the spell formulaic.
You might still, given that you invent it in the lab, raise it by a magnitude and make it a ritual with Duration: Year to make it last the longer. I would still only cast you 3 vis to cast.
Another option to increase duration, and if your sanctum accomedates it, to make an engraved circle just within the door to cast a InCo spell on and that way make it last longer. This could pose other problems. What if a non-corpus enters your sanctum? Or what if it is someone with Magic Resistance - then the spell would have to penetrate to detect them. But still feasible and could last longer.