I am again finding myself drawn into arcane mathematics and physics here about 'descent' and 'FPA'. Surely the whole point of having an independent AoA indication would be to enable a crew to ensure they are flying a safe AoA (away from any stall angles) in the event of corruption of the electronic wizardry? I do not see the need for such with fully functioning triple IRS and 'correct' IAS inputs.
Certainly trying to work out my AoA from PJ's picture, whilst wrestling with conflicting IAS inputs and possible IRU problems would have significantly challenged me! A simple vane driven display would do that with delightful ease. As with my example with the Harrier, IAS was IRRELEVANT - all that was needed was the magic 8 units from 0 kts to normal wing-borne flight. So easy!