Not only are three “enabling” aspects being sought to call a Class 2 Stone a Transition Stone but reinforcing these by carving in an integrated style perhaps has an underlying reason.
One subtle way of transitioning people’s existing beliefs and practices is to absorb those beliefs and practices into the “new” belief. In the case of Pictish-Mithraism placing the Symbol iconography for the belief in with the same surface space as the Christian Cross leads to absorption – an approach arguably more acceptable than outright substitution.
Through the transition period the Pictish-Mithraic belief can still be practiced whilst the case for the new belief is made. A very good way of working towards some form of seamlessness (at minimum, early introduction) can be seen in those Stones where the format and style of, for example, the V-Rod & Crescent and the Z-Rod & Double Disc are the same as the Christian Cross and biblical stories.
Taking this line of thought a step further it could be that those wanting to introduce the new belief recognised elements of the existing belief that could be assimilated to their advantage.