Deedle, you're correct that as soon as HMRC got involved Murray should have immediately wound up scheme at Rangers, however if it was 2008, by that time damage was largely done as only around £6m was paid into EBTs from 2008-10.
I've only ever seen that HMRC first charged £38m, ie. they immediately "grossed up" the figure, so if they did this in 2008, even without penalties Murray/Rangers were likely still unlikely to be allowed to do a deal.
IMO, if HMRC had only been looking go recover tax unpaid on EBTs ( not grossed up ) with a nominal interest charge, then a deal would have been done of some sort and they would have recovered all tax due. It is sad to this day why no politician or journalist have asked why we were pursued so aggressively for an inflated figure, which was never likely to give us a chance of meeting.
As a foot note, the financial, benefit to Rangers from a football point of view of this scheme is further reduced as £12m of EBT contributions were to Murray(£6m), other directors and non playing staff.
As you previously stated Murray had the audacity to charge Rangers yearly some £0.6m for managing the scheme, so from your figure of £18m, they have gained possibly as little as £8m from a playering point of view.
This during a time 2001-2010, Rangers paid total wages plus EBT contributions of £330m from turnover of £573m. Draw from this what you will, my conclusion is that sensible budgeting by Murray would have led to no less in achievements without the horrific cost of 2012.