All of this is described in the Scroll enchantment on p. 57 of Magic.
Reading a scroll aloud, to make it cast its spell requires that you be a mage. That's a shorthand which means you have some level of the Magery advantage.
You also need Accented or better comprehension of the language the scroll is written in; scrolls seem to be possible in any language.
There is no skill roll required to read a scroll unless the spell is one that is resisted. Then there is a roll against the skill of the mage who wrote the scroll. That is what will take modifiers for low or high mana. If the mage reading the scroll has Accented comprehension of the language; the roll against the writer's skill is at -1. There's an inconsistency in the description of the Scroll enchantment: the first paragraph says you need Accented comprehension; the second allows it at Broken for resisted spells, with a penalty of -3. I'll report errata.
I don't see anything that indicates that high mana allows non-mages to read scrolls, and there's an FAQ entry that argues against the idea:
4.7.1 Can Mage-Only items enchanted with enough Power to be considered "always on" be used by non-mages?
If a "mages only" item would produce an ongoing effect on the user (e.g., Flight), and has Power enough to reduce both its cost to cast and its cost to maintain to 0, then it will generate its magical effect at all times. As such, it is irrelevant who uses it. On the other hand, a "mages only" item that actually empowers the user to cast a spell (e.g., Deathtouch) will only ever work for a mage even if energy cost is reduced to 0. When in doubt, ask yourself the following question: "Does the item effectively grant the user an advantage, or does it just let him cast spells?" If the former, then anyone can use it once cost is reduced to 0; if the latter, then it remains "mages only."
This isn't exactly the same question, but it is quite suggestive.