As others have said, devil is in the detail with something like this.
For example, arguably, the valuation date (which is the tax point) may not be in 2012 in the circumstances as described.
A pragmatic approach, if the land is to be sold and the value is relatively low, might be for a person to simply assign a negligible value to it at date of acquisition, and therefore they would have a relatively large gain for CGT purposes at the time of sale - they'd pay CGT on all (or nearly all) of their cut of the proceeds, but not be getting into any conversations about 10+ years of interest on a CAT back duty settlement.
The likelihood of anyone in Revenue looking at it in the first instance, or taking issue with it in the second, if tax is basically being paid on the whole shebang, is vanishingly small.
Seek professional advice OP, a couple of hours of the time of someone who knows what they're at, would be worth it for peace of mind and minimization of risk.