Excellent post, Gyrth, very interesting reading. Your point about the 'bretwalda' being the foremost warrior in the land, and not necessarily the overlord, is a good one. An overlord can expect loyalty from those under him, whereas the foremost warrior should expect that he will have to prove his power, when push comes to shove. As you say, Brunanburh is an example of someone who would have some sort of power over the whole of Britain proving that his army was the best, and this is what the treaties recognised. But working out the exact nature of the power of the many lords and kings in Britain in this period is a vague and inexact science.

On a side issue to your main argument, you say of Athelstan that "He was the first King of the West Saxons to be accepted unconditionally by the Mercians, as his crowning in the two different lands were independent from each other." It's certainly the case that there is no evidence of problems for Athelstan in adding the crown of Mercia to that of his father's kindom of Wessex. But, reading between the lines, the transfer of power was arguably not as smooth as we might assume.

Since the death of ealdorman Ethelred of Mercia in 911, Aethelflaed his wife (and King Edward's sister) had taken over the running of Mercia. In fact, she had may well have been in control before, as illness and infirmity claimed her husband. She was active in supporting her brother's campaigns - she fortified burhs and attacked the Danes, and could well have brought a number of Welsh lords under her control. A plausible explanation for her assistance to Edward is family loyalty, and of a sense of a common aim with her brother. But it would have been clear to her that she needed goodwill from Wessex and she must have known that a failure to co-operate would have left her very vulnerable. This is made clear by the fact that immediately after ealdorman Ethelred's death, Edward claimed the southernmost Mercian lands, including London and Oxford. These were only on the periphery of Mercia, but a clear sign nonethless of the power of Wessex. So Athelflaed's loyalty may not have been loyalty at all, but simply expediency. It's impossible to determine that from the sources, but I just point this out to say that the situation might not have been quite as cosy as the histories normally tell us.

Similarly the accession of Athelstan to the crown of Mercia might not have been automatic, as some have suggested. Shortly after Aethelflaed's death, her daughter Aelfwyn was removed to Wessex and is not heard of in the sources again - though given the scarcity of sources, this need not lead us to assume the worst. Given that her mother had ruled Mercia for some seven years at least, it would clearly have been possible for another woman to take over. But of course Aethelflaed had ruled first with her husband, and gained much political experience, and an advantage her daughter did not have. Whatever the reasons for Athelstan becoming ruler of Mercia, sometime around this time there was an assertion of West Saxon control in Mercia, where shire boundaries were imposed in disregard to the old Mercian divisions.

So the West Saxon sources may be failing to tell us of a slightly more troublesome takeover of Mercia than we have assumed. Since we don't have any Mercian sources for this, this is all speculative, but I think we should be cautious of assuming a peaceful takeover just because we don't have any evidence for it.