Justin, I believe alternative views to Andrew’s have been expressed on this forum in the past, certainly in relation to the IHT position and the indexed uplift. You might like to look at this thread, which I started back in 2017: https://trustsdiscussionforum.co.uk/t/waiver-of-indexed-increase-in-debt-transfer-of-value/4947. I don’t think anything material has changed since then.
As regards the ability to waive the indexed increase retrospectively, I suppose it may depend on the wording of the loan, as to when the increase becomes due. A fairly typical wording requires payment “on demand” of a sum equal to the greater of the original debt, and an amount calculated by reference to RPI. As the requirement to pay does not arise until demanded (and can usually be waived under the terms of the Will), and as the indexed amount cannot be calculated until the payment is made, it seems to me that the liability for the increased amount does not arise until demanded, and can therefore be waived at any time until then. It does not amount to a retrospective waiver.