I can only tell you that a selenium toned kallitype will last longer than an untoned Kallitype. Longer than a platinum/palladium print? I couldn't tell you. Yes, the curves are very similar. I haven't seen a change in print colour with changes in humidity, or speed. Print colour can be changed greatly by the addition of other metals in the sensitizer… or by toning with palladium, gold or by choice of developer.
Papers that are neutral, without alkaline buffers work best. If the paper has an alkaline buffer, a soak in sulfamic acid will take care of that, turning it into a great paper for kallitype printing. Rising Stonehenge would be an example. A great neutral paper is Arches Platine.
Are you just starting out in this process??