What's the use of the tax1, tax2 and tax3 columns

[joebordes]

I understand you are talking about Detail View inline edit.
I don't know, I guess it was never implemented in that mode. It is rather difficult because you don't know how many there are and you could have none selected.
Yes, it is because it is really a one to many relation, in other words, it is a master - detail module, although there are a small amount of detail records.