Michael

Interesting questions.  More questions than answers though, I'm afraid.

1.  What size are the 4" thick panels?
2.  Are there multiple cracks per panel, or only a single crack?
3.  Is the crack at roughly the same level in adjoining panels, or does it vary?
4.  Is there any deflection of the veneer with the crack running along the bending axis?
5.  What condition were the joints in prior to commencement of the restoration work?

In addition, I have copied your email and my response to Bill McDonald, who formerly worked for the Indiana Limestone Institute and now runs a consultancy practice called The McDonald Group.  He is a knowledgeable chap who might have some thoughts also.

Cheers

david