Hooray! Congrats! Be sure to take way more pictures than you'd think you'd want...you'll love looking back at them when this is all done. :) Oh and we'll be excepting to see the pictures here, too.

Any and every time the toilet is removed the wax ring needs to be replaced. They are a one shot seal. Even lifting the toilet off the seal and setting it back down is grounds for using a new seal. Seals are very inexpensive, and damage from a leak will cost far more than the few dollars for a replacement seal.

Water pressure is not an issue in a flush: the flushing action is a result of the emptying of the tank, and I assume you've checked to see that the tanks fill to the appropriate height. The only thing I can think of is that some debris fell into the hole after you removed the one toilet and that that is causing a blockage of the sewer line or is somehow interfering with the vent. Two questions: 1) What is the location of the removed toilet in relation to the others? (i.e., is it "upstream" or at the highest level of the house?), and 2) Have you done any modifacations to any of the drain or vent lines in the process of the remodel?

cplover - did you look to see if you could find the manual that goes with these toilets? You might be able to tell from that. Otherwise, there should be a contact number or email to check with the companies on these products. I can't imagine a company making a toilet that you had to remove to change the seat. Not that you do it often but from a general design perspective that would not be a good design.