Learn English – A case that exhibits all the possible conditions a subject can suffer from

A regular Google search gives about 350,000 hits for ‘indifferent between’, while a Google books search gives about 120,000 hits.

I cannot find any dictionary entries that include examples or citations with this usage, but it does appear to be in actual use, if not all that common.

As such, I wouldn’t classify it as ‘truly ungrammatical’, but rather perhaps as ‘best avoided in contexts where you wish to steer clear of potentially controversial grammar’.

In your particular phrase, I would suggest:

X is indifferent to whether (we do/he does/etc.) Y or Z

Or, even more plainly spoken:

X does not care whether (we do/he does/etc.) Y or Z

Edit to reflect edited question:

With the context now known, I would suggest recasting the sentence as follows (vel sim):

The risk adjustment should reflect the compensation that an insurer requires for bearing this uncertainty and reflects the point at which the insurance company considers fulfilling an insurance contract with a range of possible outcomes and fulfilling a liability with fixed cash flows (to be) equal options.

I've worked there; it's a hellhole; a real snake pit.