I've 'always' wondered about its etymology
Solution 1:
I think this question deserves some attention.
No answer coming will probably be very satisfying. What is involved is idiomatic English of a very old sort. The question references ealne weg which seems the oldest available form of this idiom.
The main issue with this idiom is weg. The Latin cognate, via, had similar meanings with the Anglo-Saxon dialects as indicating a route, road or means. But it is clear that in at least some Anglo-Saxon dialects weg had taken on usage distinct from simple known definitions.
Those A-S dialects had a term for "always", simble and other similar words. Obviously this is a cognate with the Latin semper. Latin and the Romance Languages retained semper, but simble seems lost in current English.
Why "all ways" survives today and simble has been lost is a question I cannot answer. But this is no isolated language incident. We have almost and already among others with similar construction. These "single word idioms" were codified when Anglo-Norse and Anglo-Norman were merging to form the language we know today. One can guess that intercommunication played a part in why always survives. But I cannot point to a mechanism that caused that.
I think it fair to say that always is good, honest English, and old in the language. What is unsatisfying is that a search for the origin of always does not meet a dead end, but a dead beginning. There is really no telling what the origin of the expression is, nor how old it is. One could guess that some grammatical function in some Northwest Germanic dialects allowed a form of "all ways" to mean the same as "always" in current English, but, now, that could only be a guess.
Solution 2:
Just an idea: in many southern German dialects, the word "oiwei"/"alleweil" still exists. Sounds very similar to and has the same meaning as "always", and it has been in wider use for centuries. It literally translates to "all whiles", which would put it in the temporal context the English word lacks.