If the first book in a series is currently ineligible for a countdown deal (rookie mistake, did a free offer day), is it against Zon policy if, in addition to publishing the sequel as its own book, one were to create a bargain bundle (priced slightly less than the two freestanding books would be if purchased separately) and put the bundle on countdown?
I would think you could do a Countdown with your bundle. Each book has its own separate Select period. Of course, the parts would have to be exclusive to KDP in order to put the bundled version in Select.
I don't do very short works (I don't know how short yours are), but I price my novellas (just under 100 pages) at $2.99 and my full-length novels (over 250 pages) at $4.99 and $5.99. I would think for a high-quality, full-length work you shouldn't go under $3.99. Sure, the 99c ones sell a lot, but because of the royalty differential, you'd have to sell almost 6 times as many copies at 99c to make as much as you'd make from the same work at $2.99.
I think that $2.99 is the lowest one can go and still be taken seriously. This may not be true of genre fiction (but does anyone take that seriously?). I'd scan the top 25 or 100 best-sellers in your field and price at the top of the lowest quadrant.