Luckily for me your opinion doesnt matter lol
The High Court ruling said that it wasn't critical, since there were other problems with the NoT in that particular case. I think that means the High Court opinion is only persuasive precedent. It isn't binding, but that just makes things even more unclear.
The judge essentially said it looks bad but doesn't matter for the case in question.
Your problem is that you have a potential technically invalid NoT.
Say that the RTB accepts your technically true assertion that you sent both notices on the same day, then you aren't out of the woods.
At the actual hearing, the tenant may point out that they were served the notice on a different day to the RTB. In that case, your NoT gets declared invalid but later than Today.
It could get worse. Assume the RTB (and then appeal tribunal) all agree with you and you get a determination order. You go to court to get it enforced, and then the court says that it was invalid.
The core issue here is that email is instant and registered post (nominally) takes 1 day. By using notification methods with different delays the solicitor ended up serving on different days. This was the advice from the RTB at the time, so it is hard to fault him.
Interpretive Act said:
... the service of the document is deemed, unless the contrary is proved, to have been effected at the time at which the letter would be delivered in the ordinary course of post.
So, if you sent 2 registered letters on the same day, they would be deemed to arrive/be served on the next business day (i.e. in the ordinary course of post). Even then, the tenant can claim otherwise. If they can show it was received faster or slower, then they may be able to get a different service date accepted, if he can prove it.
Email and register post don't have that equal delay property.
Ultimately, you have a choice to make. You can wait and hope for the best. This might work, if the tenants leave.
Alternatively, you restart the entire process now. That has a better worst case outcome, but means you are giving up on an earlier date.
If you are confident that they are actually planning to leave, then maybe that is the best plan. But, the longer you wait to reset, the worse it will be if you then need it.