Stena Carrier seems a big improvement on Neptune Agile. Good levels of car deck space. About 700 more lane meters than European Endeavour. I would find it interesting to see the efficiency levels - Endeavour vs Carrier. Im not sure how EUROPEAN SEAWAY would be a good cover vessel for Stena at Holyhead. Her passenger capacity would be a major issue for instance, as it would also be between Belfast and Cairnryan. You forget also that Stena Line and Stena Roro are separate companies. P&O are chartering STENA CARRIER at the market rate from Stena RoRo - if Stena LINE were involved do you really think they would approve of giving one of their competitors MORE capacity? The timing of the charter also appears to suit RoRo, as did the charter of STENA NORDICA (which by then had passed to Stena RoRo) to DFDS.
If the bow doors on European Endeavour were brought back into operation, a refit arrangement with Stena could be an interesting option. Stena provide P&O with a vessel - eg Stena Carrier / Hibernia / Horizon for refits, and Stena use European Endeavour for month to cover Adventurer and Superfast X's dry-dockings.
In theory, following berthing trials, I could envisage some senario like that, as both companies could bring something to the table. Stena Horizon, I don't believe is a good refit option for Stena at Holyhead looking forward. On both occassions, she struggled. I recall in the past, European Endeavour's sister ship (prior to being delivered to LD Lines - I think she was named "T Rex") operated with relative ease from Holyhead. Naturally, her lower capacity than Adventurer was an issue, but from a turnaround perspective she was successful.
Next year, I would imagine European Seaway will be covering at Larne. Could she offer cover for Stena at Holyhead? Or operate in lieu of Superfast VII / VIII, the latter covering at Holyhead? Could Pride of Canterbury / Kent squeeze into Larne, and once duties have been completed, cover for Stena at Belfast, before going to perhaps H&W for a refit?
Stena do have a pooling arrangement with Irish Ferries, who are by far a bigger adversary than P&O. Stena RoRo will take the offer for a vessel that suits Stena RoRo. If this wasn't the case we would more likely than not have HIGHLANDER and BLUE PUTTEES operating between Belfast and Cairnryan at present! The whole point of having separate companies is that they will do what commercially makes sense for them. As for there being a pooling arrangement with Irish Ferries, I think you are confusing a pooling arrangement with a space charter at the going commercial rate. In a pooling arrangement both companies would be operating their vessels on a common schedule interchangeably with the same far structure and offers, rather than two different competing schedules!
It's just a theory, but if European Seaway can operate Larne - Cairnryan, I would think Belfast to Cairnryan would be straight forward enough.
StenaRoRo is indeed a separate company from Stena Line, but I imagine the latter could offer competitive charter rates, and likewise P&O could offer Stena Line a similar deal with European Seaway.
European Endeavour would be ideal for Holyhead, however her bow-doors would have to be brought back into operation for such an operation.
Brittany Ferries have a cosy arrangement with Stena Line. Stena Baltica is owned by Brittany Ferries, and likewise, Connemara is a Stena RoRo vessel.
With regard to Brittany Ferries, they needed tonnage and Stena RoRo happen to own some which was suitable. This, again, is chartered at a rate acceptable to Stena RoRo and if a better offer came in then the vessel would go to that operator instead. To be honest people read FAR to much into the perceived relationship between Brittany Ferries and the Stena Sphere. STENA BALTICA/COTENTIN is a vessel that was surplus to requirements at Brittany Ferries and put on the open market. Stena did look to purchase her but the purchase price was in excess of what they were prepared to pay. If the companies were that blind to taking the best commercial offer then would Horizon not be sub-charted out and Etretat or Asterion/Connemara not be brought in (and upgraded) instead?
It's just a theory. Pride of Burgundy would be an interesting option as a relief vessel for the North Channel. She has the required capacity for Stena at Belfast, and has the same dimensions as European Seaway. The only issue for Burgundy operating from Larne is of course her rather slab sided design. Wind could pose a problem, she has in the past made contact with Seafrance Berlioz. (However Dover vessels have had plenty of incidents over the years - Pride of Provence (Stena Empereur), Pride of Aquitaine (Prins Filip / Calais Seaways), most recently Pride of Kent)
From what I have gathered the incident with Pride of Kent (which has seen her in dry-dock for several months) was more down to an elaborate maneuver. A similar incident occurred in the early 2000s with Pride of Provence. The captain undertook a rather unusual and elaborate swing at the harbour entrance at Dover, the harbour wall catching the stern of Provence.
I think with the right weather conditions, Pride of Kent / Canterbury could operate from Larne, and then if Stena were interested, charter her for a few weeks to cover Belfast and possibly Holyhead. As Pride of Kent / Canterbury has mezzanine decks, her already tight deck height is restricted further (not sure by how much), which does make Pride of Burgundy the more attractive option. (I understand Burgundy, like Europen Seaway has no mezzanine decks, allowing larger trailers to be carried).
I wouldn't rule out some sort of deal on refits (both companies chartering to each other at the market rate). I would think Stena would benefit from having a very capable (if not more capable) vessel operate Belfast to Cairnryan - allowing Superfast VII to go to Holyhead to cover Superfast X and Adventurer. Canterbury, Burgundy, and Kent are all very versatile vessels, and Stena RoRo don't exactly have binders of ships available that can operate from Belfast to Cairnryan - not forgetting Stena Horizon's period cover at Holyhead was an utter disaster. Delays of two hours discharging passengers is abysmal.
Stena RoRo have a good number of vessles P&O could charter for Dublin to Liverpool, and its not the first time a Dover vessel has been used on the North Channel - to much surprise (though not to myself) European Seaway was the designated relief vessel in 2017 for Larne. I would have thought it quite a logical move by both companies.
With regards to Stena's relationship with Brittany Ferries, I personally see it as very cosy. Both companies clearly are on very good terms and have a very good relationship. (I understand one of the E-Flexers are going to BF in the early 2020s). It wouldnt surprise me if Stena don't buy a 30% stake in the company, and eventually move to buy the whole company and re-brand Brittany Ferries to Stena Line. I could envisage a scenario similar to the transition from "North Sea Ferries", to "P&O North Sea Ferries", to eventually "P&O Ferries".