Answers: 1) If you didnt have an evaporator coil, you wouldnt have cool air in the hot summer. Its not a weak link. You need to replace filters regularly, use quality filters and make sure there are no gaps in the return ductwork after the filter. If the ductwork is not sealed air tight, the furnace will suck dirty unfiltered air into the furnace. It gets on the motor (causing bearing failure), it gets on the blower wheel (causing reduced air flow) it gets on the secondary heat exchanger (causing reduced air flow) and it gets on the cooling coil (and youve seen first hand what that does and costs. A box of filters and a tube or two of caulk is certainly cheaper than $400 in repairs and whatever extra it has been costing you to run a furnace that wasnt operating efficiently.
2) No, the pads dont last forever. Probably just for the cooling season and then needs to be replaced. Find out from your technician or from
3) As far as the cleaning, it will stay clean now till you let dirt back in. Disenfectant? Probably wont last much longer that a few heating or cooling cycles. But it certainly doesnt hurt to clean and disinfect at the same time. You need to be trained in your spraying technique. It takes 4 years to get it right. :-)
Bubba Calgon's website.