dunkles å in Austro-Bavarian

[Io]

Any clues which [a]s have transitioned to dunkles å (I'm specifically not indicating any phonetic values to it as it seems to vary) and which have remained [a]s? Obviously notwithstanding the 'ei' diphthong and a from oa, I suspect when speakers are saying words with [a] while speaking in their dialect they're code switching, for example saying Ahnung as in Standard German, then again I've no idea if its [a:] is supposed to become a dunkles å.

[Jipí]

I don't know the historic details, but I'd look up Verdumpfung. From reading up a little on Bavarian MHG for my BA thesis¹ (though I only read up on diphthongization and its orthographic representation), I gathered that it applies to earlier MHG â /aː/. If Bavarian also had lengthening of vowels in open syllables, this may explain why there's all of /a aː ɔː/ for StdG /a aː/. But as I said, I'm no expert, this is just an educated guess.

¹) Reiffenstein, Ingo: Aspekte einer Sprachgeschichte des Bayerisch-Österreichischen bis zum Beginn der frühen Neuzeit, in: Sprachgeschichte. Ein Handbuch zur Geschichte der deutschen Sprache und ihrer Erforschung, 3. Teilband, hg. von Werner Besch u. a., 2., vollst. neu bearb. und erw. Aufl. (Handbücher zur Sprach- und Kom­munikations­wissen­schaft 2), Berlin 2003, S. 2825–​2888: http://www.degruyter.com/view/product/14296 (01.02.2013).

[ebilein]

Not sure, just something I read some time ago: [a] remained when followed by a diminutive suffix (so for example, something made of glass is "aus glås" with å, while a glass of water is "a glasl wåsser" with [a] in Glasl; Madl (~Mädchen) also has [a]; also - at least in my family: "a schnåps" but "a schnapserl", "a kåtz" (~Katze) but "a katzerl" (~little cat).

[Io]

One more question but this time about consonants: how does the voicing/devoicing correspond (if at all) to the High German consonant shift?! Is the 'd' in Dag and Deifi retention of the original d or is that a later development? Likewise the fricativisation of b. On the other hand it seems the voicing/devoicing is also due to assimilation.

[Herr Dunkel]

Is the 'd' in Dag and Deifi retention of the original d or is that a later development?

Later development.
Edit: more specifically C[plosive] > [+voice] / #_