I don’t think the East has gone as far as the West in defining who exactly confers what and when in Sacramentology.
In the Eastern Catholic Churches, there is an understanding that the Sacrament is known to be confirmed when the two give their consent and the priest imparts his blessing.
So why ‘tinker’ with it or ‘what more to we really need to know’
So yes, an Eastern marriage witnessed by the deacon alone is certainly invalid due to Form (to use the Roman terms) and to the Eastern understanding of the Mystery.
But that does not mean that the Eastern Churches, either Eastern Catholic or Orthodox, would not deem a Roman Marriage witnesses by a deacon alone as being invalid.
A Roman couple married in front of a deacon would certainly be accepted by any Eastern Catholic priest as fully married, and in most cases by the Orthodox as well. I’m sure that certain Orthodox priests might perform a conditional Marriage in cases of conversion, but that really isn’t the norm.