The "thingys" you refer to are called "P-clamps" and are available at most hardware and home improvement shops. They work after a fashion but tend to slip unless you can butt them right up against the dropouts. If you use them be sure to get clamps with a plastic coating on the section that goes around the seat stays. This is critical with Al or carbon stays.
There are racks that clamp directly into eyeless dropouts and are held in place by the QR skewers. I believe Adventure Cycling sells them through their on-line shop and equipment catalogs. None of this solves the heel-clearance problems that accompany short chainstays and big panniers.
Seatpost racks are very limited in carrying capacity (usually 15 pounds max.) and aren't very stable as they tend to pivot around the seatpost if heavily loaded no matter how you tighten the clamp. You mention that even your reinforced rack was cracking after a trip.
In order to make the gears work and not have to replace everything, I bought a little switch at the bike shop that is attached on the handle bars.
The switch you refer to was offered as an aftermarket device years ago to allow STI double shifters to work with a triple crank. When used, it changed the cable's effective length so you had access to any two chainrings at one time via the shifter. They effectively disappeared when both Shimano and Campy introduced STI/Ergo shifters that were designed for triple crank use.
You'll be astonished at the contraptions some people get by with.
You are certainly correct about this!