Correct!
In C64 mode, the 1570/1571 default to 1541 speed so will be no faster. There are some utilities that let you load a C64 program while in 128 mode (thus getting higher speed) then flip into C64 mode to run, but they're limited to single-load programs so not very useful. I've never seen benchmarks, but I think the 128/1571 combo is a little faster than an Epyx fastload cart.
That's odd, since any cart that contains C64 autoboot code should automatically flip the 128 into C64 mode. Not familiar with this "viper" though.
No idea on that one. The 80col key on the 128 is in the up position, right?