Are there any connections between ash wednesday and the worship of Agni or Odin? When did the oddly pagan-seeming tradition of rubbing ashes on the forehead become an accepted christian practice?
Etymologically it seems acknowledged that the word Ignite comes from Agni, meaning that there was an obvious cultural intermingling of latin and sandskrit at some point, right?
Was there a way that the germanic people would recognize Wodensdag? Is there some kind of connection there? If vikings were visiting asia around 1000, or mixing culturally with vedics in europe a thousand years earlier, does any of that Ash-wearing practice pass around between them?
You seem to be hypothesising a number of 'connections' common among those (on both religious sides) who want to trace Ash Wednesday to a pagan Nordic origin.
Latin ignis (fire) is related etymologicaly to Sanskrit agni, but only because both are Indo-European languages and this root appears in many other IE languages. It does not suggest any cultural intermingling of Latin and Sanskrit peoples. Nor does it suggest that Agni as the Vedic deity of fire has anything to do with the use of ash as a sign of protection from Odin. Parallels do not make (in the absence of other evidence) causation and origin.
While it may indeed be the case that northern european practices of using ash influenced or interacted with emergent traditions within Christianity, the symbolism of ash in the Biblical literature as a sign of repentance and mourning is well-attested, and this is the meaning important and imported into Ash Wednesday rituals. You see that both in the 8th century Gregorian Sacramentary and in the 10th century comments of Aelfric.