On your climax SOC level, that debate may be an "organic" cropping systems versus a "full-on nozzle-head" one?  In the FON case, the benefits of advanced IPM and complex crop rotation move the farming system to a lower pesticide/herbicide level, I am also willing to speculate the green coe-evolution of BMPs may include a number of "accelerants, elixirs, and inncolulants".  There are literally $Billions going into living cultures, biome feed stocks, and other product designs that may indeed have positive soil health/biome effects? Me, I am kind of a Johnson-SU Bioreactor to produce a local landscape soil sourdough-like biome kick-starting treatment.

For SOC carbon credits, the inventory zone is the top-most 300mm (12 inches) of the profile.  The deeper SOC stock is not in the sequestration credit design now.  But the deep stocks will be important if a bit of attention to establishment of their spatial baseline is made now.  These are more persistent as deep SOC increments and might act as a "year-to-year" buffer for rescue tillage in the topmost 300mm?  The greater persistence of deep carbon might receive a premium for carbon credits derived from deep soil carbon sequestration.

On the persistence thing as well as emergency tillage, it is part of the carbon accounting.  Right now the way the enrollment processes are moving, the approval is based in a Soil Carbon Conservation Plan (SCCP)  that is processed thought a farm/field scale carbon model like COMET-Farm to guesstimate likely outcome(s), aka the additivity test.  For the carbon model versus SCCP cropping system, there is a requirement to have five or more years of farm/field inventory of detail to drive the long-term model and the models.  Therefore the no less than five years of detail; ten or twenty are better.  

Personally I believe the proper way to the SOC and its valued "flux" inventory is to consider either the Veris Technology iSCAN or the AGCO Precision Planting SmartFirmer SOC sensors.  A row unit is around $15k +/-.  Either sensor system offers real-time feedback for direct ROI via VRT adjustments versus SOC levels.  These continuous direct-contact soil sensors provides a SOC estimate at the GPS moment.  Calibrate this relative SOC map with 2.5 acre grid point samples and you will have valid SOC map and other sensed-features in your PA strategies. I feel fairly confident that spatial SOC stock will be a major proxy for soil health management strategies.   

And one point that ag-policy types might help with is the topic of "additionality".  Additionality is a carbon credit certification that the project or process offering a carbon credit is "new" versus "in-situ" meaning no-till that has been working since 1995 will not qualify but if cover crops were added in say 2019 then the "marginal increment" of SOC traced to the cover-crop, the "new" addition, would be. We need the models but IMHO the direct measurement process will be the gold standard for SOC inventory and flux over time.

The long-term easement is an issue as are tenant/landlord agreements. There are rather significant differences in the soil carbon credit agreements that require more disclosure.  As for the SOC sequestrated, keeping it in the  soils but these are also have value and there is some looking into other methods for maintenance of in-situ stocks. Ultimately as I have pondered all of this, its time to take stock of our soil carbon inventory.