Here are a few second thoughts.
In view of Old Indic dānu- 'dew,
dripping water' and Avestan dānu-, Ossetic don
'river', I'm beginning to doubt if the original meaning of hydronymic
*dānu- was 'swift'. The hypothetical development 'swift' >
'river, current' is uncontroversial, but why 'swift' > 'dew'? But if
*dānu- = 'flowing water', all the other meanings can be
explained quite naturally. *Dāneu-jo-s would be 'like running
water' > 'torrential, swift'.
Piotr