What is this "tickover" of which you speak ? I set mine so they don't - the throttle slide closes fully when the twistgrip is closed. This was the recommendation back when these bikes were current to alleviate the surging on the overrun. ES250/2s had a spring arrangement on the twistgrip (different design to the TS & ETZ) that kept the throttle in a slightly open position, but the twistgrip could be twisted fully closed (overcoming the tickover spring) to stop the overrun surging. It doesn't take long to get used to keeping a bit of throttle on at standstill to keep the engine running.
Spark plugs are strange beasts. I don't know what causes them to die - I have had brand new ones just stop working after 100 miles, yet others seem to go on almost indefinately. I never had much luck with Champion plugs, used Bosch for years on BMW airheads, then they seemed to have reliability issues. NGKs are of the "random failures or last for ages" mould, but Densos just seem to work in my current (limited) experience. If I need a new plug & don't have one in stock, I'll buy a Denso equivalent for preference now.
Look on the bright side of this little fault-finding journey - at least you know now that your crank seals are good & the cases are properly sealed !
Put some miles on with that carb to get a bit of confidence in the bike - no hurry to sort something long-term out on my part.
Feeling a bit weary at the momemt - 630 miles "gentle touring" on the RE Classic 500 over the last 3 days - more in a separate post (when I get around to it).