Guessing there are several items that affect recovery of both...

Guessing there are several items that affect recovery of both tonnage and ounces for the MRE :
How the indicated and inferred resource reconciles with a Reserve resource (as you referred to )
How the reserve resource reconciles with what goes on the ROM pad
What recovery % occurs from the ore after it goes through the mill and each of the above are compounding

Lest say they can mine 120 vertical meters a year (really rough figures now)
And say there is 1000 oz per vertical meter (OVM) they can get on the ROM pad (think the OVM was around 40% higher than that so assuming the Indicated resource is very optimistic )
1000 * 120 M * 90% recovery is 109K oz from NN
at a grade of say 6 grams a tonne recovered from NN that is 109/(6/31 ) = 563K tonnes of mill feed
So say 1.8Mt of additional mill capacity (TPA reduced a bit for the harder NN ore )
1.8Mt * say 1 gram recovered (top of NN is open cut and higher grade ) open cut is 1.8MT*( 1/31) = 58K oz
109K oz (from NN above) + 58 K oz = 167K oz per annum
(guessing the '2.45 mt and 3.25 Mt" you stated are meant to be 2.45mtpa and 3.25mtpa )

So maybe they can achieve the OZ per annum based on the above calculation and that would be a 10 year mine life based on your 1.7 M oz .?

I wonder if your presupposition is correct that " SPR's approach is very optimistic on both production and mine life as it does not allow for recoveries being less than 100%'"

Maybe SPR is pulling forward the higher grade ore in to the Mine plan to get the ounces per annum needed so over and above the loss from recovery , and at this stage is not too concerned of ozs tail off after say year 7 as feel they will find the necessary ozs in the interim and some of the lowest grade ore will likely never be mind that is in the resource (which pulls down the average grade overall) ?

(I do not have any actual underground mine experience and even if I did would welcome to be corrected )