Could European Causeway not simply replace European Seaway (after modification of stern & cow catcher)? She is a freighter and replacing a freighter with a freighter shouldn't impact on passenger experience.
Regarding European Highlander, I imagine she could be centralised on just taking passengers with cars, focusing largely on freight. Naturally, berthing mods would be required.
P&O have worked very hard this past year. The dropping of Troon and the costly HSC Express, has allowed P&O to cut costs, also helped by the shorter crossing times and the more frugal nature of their ships vs the Superfast twins. However, like you said Steven, this goes completely out the window. Stena even market these new ships as the most efficient in the world!
If P&O moved the Highlander and Causeway to Dover, replaced them at Larne with vessels more frugal than their new Stena counterparts, then their ability to compete will be much better. However there is still the capacity increase - is this sustainable? Stena are going from 2000 to 3000, a large increase. To put this in perspective, Pride of Rotterdam has 3300 LM, and she can gobble up allot of freight.
It'll be interesting to see how Stena's running costs compare with Ulysses new running mate - Somehow, I see the Stena vessels having a considerable edge over their IF counterparts. The new IF vessels appears to have more decks = more weight = more fuel burn. Stena could theoretically replace both vessels at Dublin, and undercut Irish Ferries, like the scenario which P&O will surely experience at Larne.
I am a little sceptical of DP World offering P&O two new vessels for Larne, but if Highlander and Causeway could be shifted to Dover to replace Burgundy and Seaway, this could be an option. Remember, Seaway and Burgundy have had much tougher lives than their NI counterparts. The Japanese twins are in pretty good shape and if they were given a good dry-docking and refit (mechanically), they could easily operate out of Dover for 10 – 14 years.
I wonder would there be much of a demand (or if its possible) for a P&O Warrenpoint - Liverpool service? With the right ships could they restart a Fleetwood - Larne service?