You can definitely make a siyum if you learned in English, since it is all torah study.
As for reading silently, I asked a shayla and was told that if one is learning in-depth and reading silently helps him understand it better, then he should do that. There IS a mitzvah to know and understand halacha, as is brought down in the sefer tanya, and this mitzvah is even greater than oral torah study. But from time to time he should enunciate the words of Mishna/ Gemara with his mouth, like he’s explaining it to someone else – which may help him understand. It’s definitely MUCH, MUCH better to learn aloud, and this gives Hashem tremendous nachas.
It says in the Tanya that if one recites words of Chumash without understanding a thing, he is yotzei talmud torah. I also heard from a Chabad rabbi that by Chumash you must say it, and by gemara and things like that (Oral Law), you must understand it.
I also heard from my congregational rabbi that if a person is just thinking in learning, this is a very good use of time, as we learn in PIRKEI AVOS that if a person sits by himself and learns, the Divine Presence is with him, and even if he’s just thinking, this is also good – but saying is MUCH better.
As for Tisha B’av – let’s say that a Torah thought pops into your head; are you obligated to chase that thought away since it’s assur?