Swallowing your pride (and no doubt hearkening to the wise counsel of Lanfranc), you acquiesce to everything. The abbeys are a simple matter: you have the wealth, and so does Matilda: you both can supervise a much-delegated task, and raise up two mighty abbey churches which will impress the whole Christian world for generations to come: they will be fitting monuments to your piety and greatness. Your humility will be remembered as well as your pride. The pope's fealty is easy to deflect, with many delays and excuses: you counted on this, so that you could play for time. You have no actual intention of ever turning Normandy into (effectively) one of the Papal States.
The war in Maine goes well enough, especially after Herbert dies, and your son Robert is now the titular heir, by virtue of his betrothal to Margaret of Maine. And when the girl inconveniently dies as well, you can still say that no one else has a better claim on Maine than your house: so you are count of Maine until your son comes of age. Most Manceaux are ambivalent on this. You are more powerful than the bickering sons of Anjou, or any Breton (Conan) with ambitions in the county; so with your troops in possession of Mayenne and LeMans and many other fortresses, and with legality on your side, you are easily able to dispose of any counter-claims that arise.
(If a few people must disappear conveniently, and nothing is ever proven linking their disappearance to you, then no harm is done: a prince must make hard, personal decisions at times, for the good of his realm and people. You are no worse than others have been before in your position; and a limited amount of judicious removal of troublesome rivals is sometimes - regretably - unavoidable. Historical note: All that need be known in this regard, is that duke William had a rival for the county of Maine in one Walter of Mantes, nephew of king Edward by his sister Godgifu, who had been earlier the wife of the count of Mantes: Walter had come into his patrimony as count of Mantes, and because he was an uncle of the deceased Herbert - though on the female side - he also claimed Maine as his by right of inheritance. This was not a wise move, because William's need for Maine far outweighed Walter's in political terms, a fact the young count of Mantes seemed not to appreciate. Perhaps his wife was the over-ambitious one and drove her husband to rebel against William: they seized LeMans, but were soon forced by William's army to surrender. Conveyed to Rouen, they soon disappeared from public notice and were never heard of again.)
Your maneuverings in Brittany are a bit more problematic: the situation is harder for you, because the Bretons are more divided over legitimate claimants. Hoel is popular, but he is only one of several counts within the "county" of Brittany, but he doesn't appear to be as popular as Conan: the main trouble-maker, of course, is Conan. His chief vassal is Rivalon of Dol, who also is a vassal of yours; that gives you a possible lead-in. You have learned patience, and proceed with Brittany using guile rather than open force. Until Conan can be maneuvered into your power, you must be very cautious not to outrage the Bretons by any overt attempts on their rights.
A windfall occurs, something so unlooked for that it must be an act demonstrating God's favor of you: a messenger comes from Ponthieu with the news that count Guy has Harold Godwinson and his friends in close confinement at Beaurain castle. (Harold Godwinson is the most powerful and popular leader in England: his father before him was virtual subregulus for king Edward, and Harold has been in the same position since his father's death way back in 1053.) It seems that the earl of Wessex was out boating in the Channel, ran into some foul weather, and was forced ashore near the Somme. He was taken with all his men, and now count Guy has them as his prisoners. You send messengers to your vassal the count of Ponthieu, ordering him to deliver up Harold and his company. Count Guy sends a defiant message back: you can either ransom them, or try and take them.
Pay a personal call on the count of Ponthieu.
Invade Ponthieu and seize Harold.
Agree to ransom Harold and his men.