Here's a critique assuming that you used 5th edition throughout:
(1) Multiple Casting doesn't require Finesse rolls in ArM5, so requiring a Finesse roll for a spell's effect should indeed help to keep the trickyness-modifiers to its level low.
(2) The base individual of ArM5 CrTe spells creating stone is one cubic pace, so the level of your spell would be computed thus:
Base level 3 (create stone), +2 magnitudes (more than 10 times base individual size created), +1 magnitude (R:Touch), +1 magnitude ( D:Diameter), +? magnitudes (tricky effect, with several spells cast simultaneously allowed to support each other).
So the following spell might make more sense for you:
CrTe 10 Temporary Corner Stone
R: Touch D: Diameter T: Ind
As per Wall of Protecting Stone except: makes a wall of granite up to 4 paces wide, 2.5 paces high, and 1 pace thick.
Note: A caster who has mastered this spell for Multiple Casting can create, with a successful Finesse roll (Ease Factor 3 plus 3 per additional copy of the spell cast) and sufficient space, several walls that will support each other.
(Base 3 +1 Touch, +1 Diameter, +1 Size, +0 (SG or Troupe call) Trickyness due to required Finesse roll)
I wouldn't require that the spell be altered in order to allow multiple castings to form corners. That's what finesse is for; "manipulating spells and performng special feats with them" (p.65).
You could get a very similar affect by just locating the walls in such a manner that the one you do not wish to be toppled is supported by the other (preventing toppling in the direction that you fear most).
I (also) agree with Beringar's treatment of the size modifier