These are really good questions.
A few things have too be qualified to be able to answer your questions, since there is a lot of varialbles to all this coax length stuf.
If the antenna is matched to the coax, then it is said to be flat or have no SWR. In this case adding or subtracting coax is not a big deal.
However if a significant amount of coax is added to a perfectly or not perfectly matched antenna, then the SWR measurement will show a lower SWR and a wider bandwidth. The reason the SWR will show a lower value, and the bandwidth will show to be wider, is the added loss in the coax. As the coax is made significantly longer, then less energy will gets to the antenna and less energy will return as Reflected energy or SWR. This is a old trick to make antennas look like they have a very good SWR and wide bandwidth.
This can be proven with a antenna analyzer or SWR Meter, by measuring the SWR and bandwidth right at the antennas connector. Then taking say 50 or 100 Ft
of coax and measureing the SWR and Bandwidth again. You will see a differance. And this is a normal thing that happens with all antennas.
When a length of coax is slightly made longer or shortened, and diffferent SWR is measured, this indiactes that the coax is now radiating energy from the outside of the shield. So in this case there will be a length of coax that will trick or indicate the SWR meter into showing a lower SWR. But this also indicates that the antenna end coax is not properly grounded, somehow. Or even the design of the antenna will cause such radiation on the coax shield.
I think you have a good handle on it all. But I have NOT read any one book or heard any one person tell me how all the different variables and coax lengths are supposed to work and why. Welcome to the club.
If you have a antenna analyzer, its great to go out and experment with all the different antennas and such. Adding coax lengths and seeing what happens will be a some fun. But document your measurements and list the antennas configurations, so that you won't have to repeate measurements. And you will learn a lot.
Obviously I have not covered it all, but neither has any one else!
Jay in the Mojave www.a1antennas.com