You cannot.
After more playtime trying to find opportunities to demand this, and investigation online, there does not appear to be any way of demanding an AI move it's armies away.
This sounds like it would be under the "Discuss >" category on the Diplomacy screen alongside the other demands (like "stop converting me" and "don't settle near me"), but I have not seen it, nor has anyone that I have been able to find.
After a little more research, I still don't have a definitive answer, but here's my semi-educated theory:
Like Civ V, the base number of religions per game is tied to the map size, according to the following distribution:
╔════════╦═════════════════╦═══════════╗
║ Size ║ Major civ count ║ Religions ║
╠════════╬═════════════════╬═══════════╣
║ Duel ║ 2 (default) ║ 2 ║
║ Tiny ║ 4 (default) ║ 3 ║
║ Small ║ 6 (default) ║ 4 ║
║ Normal ║ 8 (default) ║ 5 ║
║ Large ║ 10 (default) ║ 6 ║
║ Huge ║ 12 (default) ║ 7 ║
║ Huge ║ 10 ║ still 7 ║
╚════════╩═════════════════╩═══════════╝
I got these numbers by actually starting a new game with each of the initial condition combos (playing as Catherine de Medici, starting in the Ancient Era). I beelined for starting a religion and checked the "All Religions (1/n)" button after doing so.
There are exceptions, though. In particular, game era makes a difference. Quoting page 133 of the game manual:
Great Prophets are no longer available when starting the game in the Industrial Era or later.
So there's no religion at all in games that start halfway through history. My guess is that this is because the game's Great Prophets all come from three eras: Classical, Medieval and Renaissance. So even if your game still has available religion slots left (like mine), you could miss out on getting to found a religion if you've progressed to a later era or all the Great Prophets from your era have already been generated. (This doesn't work the other way around, though; I was able to generate a Classical Era Great Prophet while still in the Ancient Era, and successfully founded a religion with that unit.)
There's still a hole in this theory: I loaded an earlier save file for my game, when I was still in the Renaissance Era, and the "All individuals of this type [Great Prophet] have already been earned" message was already there.
Best Answer
Range is not restricted by units in front, but can be restricted by terrain. Usually its hills and forests that can in some cases reduce your range. Consider the following cases:
Thats an archer on a plain, 2 tiles from a city on a plain with a hill in between. In that case the archer can not hit the city because of the hill between them (ie a vertical obstacle)
In this case though the archer can hit the city though because even though there is a hill between them, the Archer is on a hill itself and doesn't to shoot over the hill between itself and the city.
In this case though the archer again can not hit the city because there is a hill/forest combo tile between them. Even though the archer has elevation because it is on a hill, it can not shoot over the forest (which effectively has an elevation of 2, 1 from the hill and 1 from the forest).