Mercurial magic is phrased as to be used by wizards (and presumably elves), so I assume it does not apply to clerics. Is that the case? Basically 95% sure on this one but I'd like to avoid being horribly horribly wrong and not knowing it!
A wizard can only learn new spells when he levels up, and the table on the wizard class page (and presumably the same for the elf) indicates the maximum. If he has access to an inscription of a spell, such as a scroll that might cast it, or a rival wizard's grimoire, he may choose specifically to try to learn that spell on levelling up (but not before), and if he has no such access would have his potential spells determined randomly. True, false, or somewhere in between?
Personally, I determine a lvl 1 wizard/elf spells randomly, and allow other possible spell sources to be in addition. This gives the wizard/elf some choice as to what to learn. Note that a week per spell level and a roll are still needed to learn the spell. I.e., a wizard/elf might start with 4 potentials, but not be able to learn the four randoms, and need to seek more spells before filling his maximum slots.
Assuming I've got that part, are those randomly determined spells normally presented to the wizard like "hey, here's 3 spells you might be able to try to learn, but you only get one, so give me your first choice and we'll see if you can", or is it more like they're just presented in an arbitrary order and the first one he can learn is the one he gets? What would happen to a wizard that has "open spell slots" for known spells because of a failure in this process? Could he try to fill them in the meantime before he levels up again, or would he just need to wait to try again next time, even if he had access to different spells he didn't try and fail to learn?
A 1st lvl wizard/elf can learn more than 1 spell. A wizard can learn 4, modified by Intelligence, for example. If a wizard still has open spell slots, he can potentially learn other spells before levelling up again. He can try to re-learn a spell he failed to learn after levelling again.
Do clerics also determine their spells randomly as do wizards (and presumably elves)?
Right now, I have 1st level clerics roll 4 random spells, and those are the spells they know unless there is some glaring inconsistency with their god. I am working on a more deity-specific system, though.
The combat section refers to "roll[ing] for surprise", but under the initiative heading it mentions that surprise is determined by the judge without the use of dice, specifically that it is based on whether the characters previously were aware the opponents or not before the combat began. Is this just sort of an artifact of editing or did I miss something?
Probably a little of both. Either roll, or determine, as the judge deems appropriate on a case-by-case basis.
A critical hit is described as needing a result of 20 or the character's threat range, which I get. But then on the next page it says that this threat range shifts to the highest equivalents on a higher faced die, so a d24 for a guy with only a threat range of "natural 20" would be "natural 24 = crit". Does this also apply to smaller dice, like a d16 = natural 16 = crit, or does the critical threat range not adjust downward, only upward? The two weapon fighting table seems to indicate that this may not be the case, as it mentions a 16 on a d16 is a crit on your main hand weapon if your agility is 16-17?
Crit ranges shift upwards, but not downwards. The only explicit exception is a halfling using two weapons.
Hope that helps.