@jayjo
I have only comp kettlebells, but those with 35mm handle diameter- dunno what the iron kbs have. Usually my thumb lies relaxed on the top of the handle, but I'll have an eye on this, if I change the grip unconsciously during the workout when things get more exhausting.
@Bkb
So what's your routine if the elbow tendonitis returns? Do you take a break or do you start immediately with the exercises/high rep training mentioned above?
Most PTs advice to rest in the acute phase, then (phase 2) use different stretches, maybe ice etc - some add eccentric training at that phase. Some skip the stretches and swear on eccentric training only, others on massage (for the blood flow) and use braces in addition etc.
I've tried that all for a specific time and still haven't found a routine to get finally rid of the tennis elbow - it's very frustrating.