Can he ignore conditions from ordinance and other shots?

Can he ignore the harpoon condition?

Take 1 damage from 2 TLT shots that land and invalidate Maul removing stress from ghosts?

Are all of these things true?

What stuff does countdown prevent from happening?

Can he ignore conditions from ordinance and other shots? Yes. By cancelling all dice, Countdown causes, say, Ion Pulse Missiles to miss, and not apply Ion tokens. Unless shot by Blount, but that's Blount.

Can he ignore the harpoon condition? In the way I think you mean, yes. If he somehow already had the condition on him, he could cancel all the crit results, and prevent them from activating the condition.

Take 1 damage from 2 TLT shots that land and invalidate Maul removing stress from ghosts? Maybe? Countdown takes a stress, so it's pretty hard for him to use his ability twice (and he'd take two damage, since there are two separate attacks). I think Yorr can eat the stress (it's not quite like a Zuckuss/Maul wording), but I'd get a second opinion on that part. But without Yorr, he'd be able to evade one normally, then use his ability on the other one, and the Ghost would have to remain stressed.

Are all of these things true?  Pretty much.

What stuff does countdown prevent from happening? Anything which depends on [1] an attack hitting [2] uncancelled results. Stealth Device isn't terrible on Countdown, for example, since CD can prevent the attacks from hitting. Or it wouldn't be terrible, if it wasn't more expensive than LWF...

3 hours ago, theBitterFig said:

I think Yorr can eat the stress (it's not quite like a Zuckuss/Maul wording), but I'd get a second opinion on that part.

Yorr should be able to eat it.

Countdown's ability is written with suffering the damage being the cost to activate the dice cancelling, and only once all that is done does the stress token come in. This is as opposed to say, Opportunist, which requires receiving the stress as part of its cost.