The likelihood of any new link to Letterkenny-hey being built on either the old Lough Swilly route (via the boonies) or the CDR route (via Tripoli and Cork, as shown) is nil. Absolutely nil. The only way to do it is a new route, like suggested above via Newbuildings, new bridge across to carrigans, and via your green line. Dunno if that passes anywhere with more then seven inhabitants, but none of the others serve anywhere of much size either; thus Letterkenny itself, as a single destination, would have to be deemed to be viable in itself. I wonder if the proposers of such schemes have surveyed the number of bus passengers between the two points? That will give the best possible guess with regard to viability.
As far as the Derry Road is concerned, massive disruption, diversions or both would be necessary now to get it out of Portadown, let alone through Dungannon and Omagh.
Experience in other countries of reopenings after decades closed often produce results like this. Expect some new stations to be nowhere near the originals!