Free >>> Ready Quicksilver. Use this power only once per turn and only if you control no face-down resources.

Can the effect be stopped by replacing an opponents resource after the cost has been paid?

I think the way it's worded, it's essentially:

"As an additional cost to use this power, have all your resources face-up."

So even if you manage to get one face-down after using the Free, it will be too late since having them face-up is not a requirement of the effect resolving completely.

Nope---"using" a power is equivalent to "playing" an effect/plot twist. Once an effect is on the chain, you're already past the play and use restrictions.

Both answers are correct. To slip in a strategy suggestion, your best bet is to perform the replacement(s) as late as possible before Quicksilver exhausts. Obviously, the Reality Gem doesn't allow for combat phase replacement, but there are other effects which will allow you to replace or, in the case of some DE tricks, completely wreck the opponent's resource row such that Quicksilver will be less likely to make that second shot.

well you can also use a HaB type effect in response to the declaration of the quicksilver attack, before the attack becomes legal, then you pray that you replaced into a character. That way quick can't re-ready.

Mad Harriet is also pretty good for getting her skanky paws into your opponents resource row (as well as into their pants).

Unless she get's KO'ed before her effect goes off.

How would that be ruled anyways?

Well, since her textbox says "Only if Mad Harriet is in your resource row. . ." then the effect would fizzle if by some means she was KO'd prior to resolution of the effect.

It says:
Being in the resource row is only to use her power.

Luckily, from the Glossary:

Yep... I had to stop a match for a few minutes to look that up.

Darn Darkseid players!

Are 'Revealed' resources considered face-up while the accompanying effect is on the chain?

I'm mildly embarrassed I don't already know this.

Generally, the reveal lasts only as long as is required.

If a card is just revealed as part of paying a cost---Pym Laboratories---then it's revealed only while you're paying costs. If a card is being revealed as part of an effect---Black Panther---then it's revealed only during the resolution of that effect. If a card is being revealed in order to use one of its powers---Mad Harriet---then it stays revealed while you're playing the effect.

But even beyond that, the length of the reveal doesn't really have a big impact. Face-down resources are still considered face-down while they're revealed; they aren't briefly locations or characters, or anything like that, so I don't think there's much to gain from a card staying revealed.