Tigran, I am a subdeacon in the Armenian Church, and would be glad to help you in any way I can as you prepare for your ordination. Although from watching your YouTube videos, I think you're going to be just fine. What wonderful work you've done already!
To answer your questions:
Subdeacon in the Armenian Church is kind of a no-man's land in between the 4 Minor Orders and the Major Order of the Diaconate. Technically, it is a Minor Order, but in practice and title, it's far more. Its role has expanded a bit in America, and is becoming more frequently bestowed upon advanced altar servers. I'm in the Eastern Diocese, so I'm not sure exactly how my experience here is different from how Hovnan Srpazan does things out west. But I'm sure there are similarities.
Ordinarily, the role of a subdeacon vs. an ordinary tbir (clerk/acolyte) is as follows:
-Subdeacons are allowed to chant the Gospel reading for the day, and in some places, may chant the Old Testament and Epistle readings as well. It depends on your local custom.
-You may already have learned censing, as it seems that's becoming more commonly taught to acolytes. You're going to need to know the censing patterns and practices for the Badarak, morning service, and Hokehankist.
-In the Eastern Diocese, and it may be more strict in the Western Diocese where more people are Armenian-speaking, subdeacon is the first level of altar service in which at least being able to read Armenian characters is required. Believe me, you will get absolutely nowhere in your service if you can't read Armenian. It severely limits your ability to utilize all of the service books, and makes it impossible to chant the Gospel. I know a few subdeacons who have gotten around this with transliteration, but it's less than ideal. You will not be able, in any circumstance, to move beyond being a subdeacon without being able to read Armenian.
-A subdeacon may carry an empty chalice, but not once the Eucharist has been placed in it.
-Subdeacons are expected to have more knowledge of the non-Eucharistic services, so you'll need to build confidence with the Evening and Morning services. You will also need to be comfortable with the Baptism, Wedding, and Funeral services. I was never asked or expected to participate in those until I was a subdeacon. Weddings are easy if you know the Badarak (Liturgy), baptisms and funerals are a little more tricky, but once you do one or two, it's really, really straightforward.
-You will be expected to take more of a mentorship role with younger altar servers. I'm sure you're already doing this, but it helps to be reminded that your increased rank means more of a responsibility in this regard.
I'm sure you already have a great relationship with your pastor. Meet with him one-on-one as frequently as you can between now and December, and even after that, and just practice and much as you can. I'm preparing for ordination to the diaconate right now, and meet weekly with my priest to go over things, ask questions, even just talk about things, and it's incredibly helpful.
Most of all, just remember that this is just another step in your service. Don't be pressured or intimidated by the extra knowledge and responsibilities, and if you make a mistake here and there, no worries. That's how you get better next time. If I can do anything to help you, feel free to send me a PM with your email and we can get in touch.