With a POD not before the deaths of Aethelberht and Saebert in 616 AD, is it possible for paganism to win out in England long term? Could a pagan England even survive in a largely Christian Europe?
Most probably not.
Inner pressure was probably an important factor, in spite of what Bede argued. While the post-Roman urban network in Britain was significantly weaker than in continental Europe, Irish missions and monasteries presences managed to maintain (and probably strengthen) a popular Christianism, while topped out by Anglo-Saxons rulers (maybe due to the political opposition between native and germanic entities that didn't really helped much to cultural mixing).
Anglo-Saxon Britain wasn't isolated from Europe either and ties over the Channel reappeared around the late VIth century (for what concerns eastern Britain, it can be argued they never really disappear for the western parts) : the presence of strong Christian entities on the continent (especially Francia for what concern southern kingdoms as Kent, Wessex, Sussex and East-Anglia, that were probably the most Christianized already) and their growing influence provided a diplomatic motivation, and therefore everything related (such as trade).
A bit more on the ideological side, Christian conception of the world allowed the strengthening of the germanic royal position, from a mix to priestly/warrior duties to a kingship associated to overlordship over a people, as God had on Earth; as well managing to swallow up more easily the Brittano-Roman population.
Giving these, both inner and outer pressures would probably be too strong to allow a long term pagan Anglo-Saxon ensemble.
It doesn't mean that it could be delayed : Aethelbert's conversion didn't launched a deep christianisation of Anglo-Saxons and pagan resurgences happened regularly.
With a Frankish-screw TL, maybe you could have a Christianisation of Anglo-Saxons happening rather in the VIIIth century, or even later.
But the lack of Anglo-Saxon political, but as well ritual, unity is going to be the main obstacle over this, when it'd come to face aformentioned pressures : if not Franks, then whoever would take their places (Goths, Gallo-Romans, etc.)