I can't say for sure about the mixing of the stain & grain filler, but gut feeling is there could be an issue with water & oil.

I'll be interested to see it sanded back. One observation though, is that is the tightest looking grain I've seen for Ash so you may not get a very strong contrast between the grain and non-grain fibres. You'll know better once it's sanded.

Watching with interest (still ).