I doubt there would be case laws on these points, since the CFR's are rules, not laws. Because they are rules, the interpretation would almost be entirely up to the PTO, unless a specific interpretation was found to be violating the laws.
I would agree that IBR is OK per se in a provisional app, but I would definitely say IBR cannot be used to exempt the drawing requirement in a provisional. As I stated earlier, the IBR rule is different from the drawing requirement, and shouldn't be interpreted as allowing a complete exemption of drawings. Yes, one can amend the Nonprovisional later to add drawings, based upon what is in the W/D, but EVEN assuming that one is capable of completely describing the invention in the W/D alone without any drawings, the drawing requirement still stands.
MPEP 601.01(f) [R-7] Applications Filed Without Drawings, appears to provide some narrow exceptions to where drawings are NOT required for filing, IBR is not among the exceptions. (AND, applicant face 2 hurdles in the determination of whether drawings are necessary, if no drawings are filed. OPAP 1st determines, and if OPAP determines NO drawings are necessary, specific Tech center may later override OPAP and determine that drawings are necessary, which would force PTO to "reconsider the filing date issue".)
In a way, IBR's in the Provisional App is kinda like applicant admitting that YES, I'm incorporating these previous App's, which all needed drawings, but no, I'm not filing the drawings, because you don't need them to understand.
I think the key distinction is between "necessary to understand" vs. "you already have them by IBR". If you put in IBR, then you are saying to the PTO that you should have these drawings by reference, but that STILL means that the drawings are "necessary for understanding".
Then PTO will simply reply, you already have the drawings from the IBR, just file them.