One consequence of the prevalence of Katadesmos Magic is the manifestation of the Leadworker Virtue (Houses of Hermes: True Lineages, page 143) in non-Hermetic wizards who may take this Virtue as a Supernatural Virtue. Among magi, leadworking remains a secret of House Tremere. (TSE p.122)
Not that this needs to be true in this saga, but canonically it is a secret.
I'm aware of the citation, yes. It just strikes me as odd/misplaced given the prevalence of the practices that the Leadworker virtue describes. It was hardly a unique magical practice. But regardless ... I am fine adhering to that anyway.
Any suggestions for alternatives? Thoughts on the Cautious: Finesse idea?
That's because that presence is the non-Hermetic Virtue. House Tremere integrated it but hasn't shared it.
I'm a fan of Cautious with Finesse/Concentration and have used them a lot myself. I'm not sure how much Finesse stuff you'll be doing, but it can be helpful with counter-magic via fast casting.
I think I'll adopt the Cautious: Finesse option then. It doesn't sound like you disagree with me regarding Boosted Magic either. 
This is an interesting point about why it is more scarce specifically in the Order.
Yeh. Boosting seems to be only really useful in one of two combinations: paired with rituals to so you can build it at a lower level and use it at a higher one at no cost (e.g. switching Individual to Group when healing), or when you have Imbued with the Spirit of (Form). As Boosted Magic doesn't allow for rituals, that leaves out the former.
I've been considering adding a stone tower to the perimeter of the ringfort to hold the sanctum of Vorsutus. It seems normal to (judging from Covenants at least) for such a sanctum tower to still have a guard emplacement at the top but that isn't necessarily mandatory.
Just to review my previous guidance on Labs etc.
Following this I am envisioning a stone tower of superior construction along the rear section of the ringfort, likely across the fort from the main gate and with the manor between them. The covered watch tower floor on top would thus command a view of the rear of the fort and possibly the grove where the oath stone stands outside of the fort in the interior of the island.
The fortifications of Inchmore are all three stories high including the gate/quay tower, wood guard towers along the palisade, and the stone guard tower on the manor. For sake of uniformity I was thinking of doing mostly the same, though I think technically I may be adding a visible floor.
I have in mind the ground floor mostly being a studio/anteroom with a display of hunting or other trophies belonging to Vorsutus while also being a furnished social room for meeting guests and negotiating with potential clients. Near the door of this room are doored stairwell entrances leading up and down (leading all the way up the watch tower, or down to the store room). In the rear of the room is heavy door with a prominent sanctum marker overhead that leads to a stair well leading up to the private floors containing the living quarters and laboratory.
Broken down by floor & contents:
4/Top: Covered Watch Tower (Accessible from "front stair", outside of Sanctum Mark)
3: Laboratory (Inside Sanctum Mark, accessible from "rear stair")
2: Living Quarters (Inside Sanctum Mark, accessible from "rear stair")
1: Anteroom/Studio (Trophy display and social meeting room for Vorsutus, outside of Sanctum Mark)
-1: Subbasement Storeroom + Entrance to Inchmore Cave System (Accessible from "front stair", outside of Sanctum Mark)
This would provide the cave system connection stipulated for the cave system aura to have grown and now include the tower.
For free virtues and flaws I've picked out the following:
Dedicated Building
The lab (and probably the living quarters of the owner) is in its own building, shared with no one else. There are likely to be fewer distractions.)
(+1 Upkeep, +1 Aesthetics, +1 Re)
Defenses
The lab has some kind of sturdy defense, such as a portcullis or battlements.
(+1 Aesthetics)
Superior Construction
The lab benefits from distinguished construction techniques, for example, high quality building materials were used, or it was made by a master architect or a Magus (such as with the spell Conjuring the Mystic Tower). If constructing the build, its cost is doubled.
(+1 Safety, +1 Aesthetics)
The mundane labor allowance of two seasons I would like to allocate, both together, to the Anteroom/studio reflecting a very high quality social space being built there and reflected with the addition of the following virtues.
Studio
An ornate room just outside the lab proper is set aside for the sole purpose of displaying and showing off the owner's most prized possessions to guests. Verditius Magi commonly build studios in order to display the items they have crafted to potential customers (see Houses of Hermes Mystery Cults). The studio takes up more of the sanctum space, but not usually of the laboratory chamber itself.
(+1 to +3 Aesthetics, depending on the number and magnificence of items displayed.)
(For Vorsutus as a newly gaunteled Magus I think it is safe to assume he has no truly impressive trophies yet, so the +1 end of this seems reasonable for now.)
Grand Entrance
An elaborate and sizable entranceway, such as a great door or arch, an opulent lobby, or perhaps a reception room leads to the lab. This takes up more of the sanctum space, but not usually of the laboratory chamber itself. It often takes a season or more for the craftsmen to build or outfit. The sanctum marker is likely displayed prominently. The lab at least looks good from the outside, so casual observers are likely to be impressed.
(+2 Aesthetics)
I'd also like to assign two guards to the tower, both guarding the door and manning the tower in emergencies.
Guard x 2
A shield grog is on permanent duty at the lab's entrance, and may dissuade unwanted visitors from pestering the Magus or intruding.
(+1 Aesthetics x 2)
Once the details are settled I can/will write a more in depth description.
Thoughts & feedback welcome.