AS I understand it De and Do are interchangeable in Munster.

Can anyone tell me to what extent, does it not matter if I use díom or dom, den or don etc ?

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Bíonn rudaí maithe mall

Do/De are interchangeable, so are don/den. However this really means that people use do and don for both. However the inflected forms like dom, díom are not interchangeable.

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal

Same thing in Connemara and maybe in southern Donegal (if I remember well).
However in northern Donegal as far as I know, "do" and all its forms are used in all cases instead of "de".
Strangely enough, it looks like Scottish Gaelic and Manx normally don't mix up do and de (well, their equivalents)

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

And how are they pronounced in those languages?

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__̴ı̴̴̡̡̡ ̡͌l̡̡̡ ̡͌l̡*̡̡ ̴̡ı̴̴̡ ̡̡͡|̲̲̲͡͡͡ ̲▫̲͡ ̲̲̲͡͡π̲̲͡͡ ̲̲͡▫̲̲͡͡ ̲|̡̡̡ ̡ ̴̡ı̴̡̡ ̡͌l̡̡̡̡.___

Depends on the dialect... in Manx the word is "jeh", pronounced "jeh" (for once a Manx word is pronounced as it is written ). In Scottish Gaelic I'm not sure, have to verify...

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

Any idea when they started to fall together in Irish?

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__̴ı̴̴̡̡̡ ̡͌l̡̡̡ ̡͌l̡*̡̡ ̴̡ı̴̴̡ ̡̡͡|̲̲̲͡͡͡ ̲▫̲͡ ̲̲̲͡͡π̲̲͡͡ ̲̲͡▫̲̲͡͡ ̲|̡̡̡ ̡ ̴̡ı̴̡̡ ̡͌l̡̡̡̡.___

I read somewhere (Thurneysen? Stair na Gaeilge? Sengoídelc?) that they even started to be mixed up together in the Old Irish period

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne