I don't really see what you mean (it's arguably late there). Alba, as an unified kingdom of Gaels and Picts didn't appeared after that Northumbria virtually disappeared from radar (you had fifty years of coexistance, Northumbria being a Norse puppet).
Furthermore, there's huge difference between Northumbria and Mercia : demographics, importance, "economics". It seems that the kingdom was a bit poor-looking compared to Mercia or Wessex, being beaten by Anglo-Saxons and Picts alike.
If "Galles" appears as more or less united (not saying unified) as Picts and Gaels did OTL, aka during the IX century, they would have to deal with a powerful Mercia (I don't really see a reason for Gael presence in Cumbria being that a change in AS history, at least up to a possible but not directly consequential unification) and likely recieve the same anwer : fighting back, and contain raids.
That's 400 years at best from a PoD. Depending on how things evolve, the situation could be unrecognizable enough to not safely bet what would happen locally.
Vikings didn't settled Ireland or Britain's shores for plunder (while it played a great role) but for controlling trade roads as well : plundering to death a region isn't going to create new markets. If Irish Sea know a more important trade between two sides (and possibly trough Atlantic), you may have more or less settlement than OTL depending on the situation (I would think that it would be an impetus for more strong irish kingdoms, at least in the eastern part of the Islands, and succsessful campaigns against Vikings aren't implausible)
I don't think it was rationalized that far. Simply that Northumbrians had a more important focus on AS matters, and that Mercia at the peak of its power was able to deal with minor annoyances as welsh raids.
-What's not to understand? I was just pointing out that the ancestors of Scotland's Lallans speakers had to come from
somewhere, didn't they. Given how limited the time-frame of coexistence between Alba and Northumbria itself was (something that did slip my mind), perhaps I should've phrased it more as "Alba's relationship with Bernicia+Deira", since it was the Anglo-Saxons which ultimately informed Lowland Scotland's culture/language/government, etc.
-Northumbria's lack of wealth relative to Mercia seems to me to be dependant on two things post-650s (Northumbria DID reign over Mercia a spell, after all);
*The Picts raiding southward, which is something that Gogledd will likely have coincidentally in check whilst they expand (unless they achieve union early enough to be the ones
launching the raids, which spells bad tidings for the Northern Anglo-Saxons), and
*The Viking incursions they had to put up with, whereas Mercia was solidly landlocked and thus less vulnerable by comparison. Again, that's assuming the Vikings attack the same spots of Britain as OTL.
-What do you mean, "Gael presence in Cumbria"? The OP indicates that Cumbria (a part of OTL Northwest England) could be, well, Welsh ITTL. Unless you meant "Gales", in which case I think I get what you're saying in that it shouldn't alter Mercia & Co. directly. Of course, if we're talking the IX Century then we still have to take the Vikings possibly setting up a *Danelaw analogue into account (those damned Northmen seem to be a helluva wildcard for an OP like this...). One thing that could result is possibly a stronger redoubt against the Normans compared to OTL Wales IF the Gaels consolidate early enough and establish a sort of understanding with the English a la Hywel Dda (that's assuming good fortunes for Gales, or even whether the Normans enter the picture whatsoever ITTL).
-I agree that Ireland may be more predisposed to resist/challenge Norse settlement on the island, which may see either more settlers landing in western Britain or force the Vikings to redouble efforts on the eastern shore. Who knows? As far as attitudes towards the Gaels, there is some evidence for a quasi-apartheid society in Anglo-Saxon England with the Britons on the bottom of the totem pole, whereas I don't recall any such system arising in the North towards the Scots during the Medieval Period (I don't count post-1066 in Ireland, that was all the Normans' fault). Then again they didn't live there as long as the Britons did in pre-Folkswandering Britain so maybe it boils down to a question of less exposure.