A lot of people think that, there are a ton of FAA regs related to seats. Everything has to be tested in crashes. That also why it takes airlines so long to change seats to the new entertainment ones with power and everything. Your seat is your number one safety in the event of a crash. You are actually supposed to use the seat in front of you to brace yourself. It has a certain give to it so you don't run in to a brick wall when you land hard. If seats were easily configurable, every possible combination would need to be tested. Also whenever you do any work on a plane, you must have all of the supporting documentation, just think how long that would take to turn around. Document each bolt that was removed, when it was replaced, where it was..etc.. a lot of hassle.
If you are able to design one that passes all FAA/EASA/CAA/<aeronautical regulatory body here> certifications, I think ALL airplane builders will hail you as their saviour. Not to mention the airlines ;-)
The seats sit on rails on the floor, so they can be swapped out or repositioned quite easily. Usually the Supplementary Type Certificate for each aircraft plans for this.
While being true, not all Seat configurations are automatically approved for operation. The swapping itself can - as mentioned - be performed overnight.
It's actually pretty easy for (e.g.) United to add or remove rows of their Economy Plus seating. They can make that change overnight.
When United and Continental merged and began adding E+ to the Continental fleet, most aircraft were completed pretty quickly since that change can be made during overnight maintenance at the hubs.
