The "Ja" is not part of the sentence -- it's sort of a clause of its own. (Though it has no verb, so it's not actually a clause.)
You could punctuate it as "Ja: es ist weit" or "Ja -- es ist weit" if you wanted.
It's a bit like how in English, the question word comes first ("How did you find him?" rather than "You found him how?"), but still you say e.g. "Tom, how did you find him?" and not "How Tom did you find him?": the "Tom" is not part of the structure of the sentence itself. Similarly with "Ja" here.
Please let me use yea or yeah instead of yes.
Not going to happen. That's considered too colloquial for this course. We ask users to use Standard Written English.
So yeah and gonna and kinda will not be accepted, nor will abbreviations such as if u c what i mean or r u sure.
Yet it’ll accept “yeah” in other exercises.
Possibly. The sentences were created over a period of several years by a large number of people, and when a policy is set, it's not easily possible to go over all existing sentences to make sure they follow it.
If someone reports something on a sentence that accepts "yeah", the "yeah" is likely to be removed.