It's a bit confusing because it involves the words "aquí" and "donde" which makes us think of location. But technically we're not talking about the location of anything. In this case, "aquí" is the subject of the sentence, and we're describing something that happens there. We're not talking about where "aquí" is located, so "ser" is acceptable.
No, because ser is always used when you're talking about events. Even the locations of events. You wouldn't say "la fiesta está en la casa" you would say "la fiesta es en la casa" In the case for the sentence in this question, the event is "tú comes" and we're describing where it is... "es aquí". Or in the case of this awkward sounding sentence, "Aquí es donde tú comes".
Not great at spanish but it seemed to me like the phrase you mentioned meant something like 'from here on in', so I searched it in google translate (I know :P) and it turns out that adelante means forward, so aquí apparently keeps its meaning of now in that particular phrase, at least :)
There could be context where it makes sense but it sounds mostly wrong. The "it" in "es" you are getting the "it is" from is referring to "aquí" / "here" so it is redundant and wrong to use both at the same time. In the same way "Laura es una mujer" = "Laura is a woman" and not "Laura she is a woman"