Use a tachometer to see what rpm the engine is running at high idle. Toro’s spec is 3800 rpm. What do the rpm’s jump to when it increases for 10-15 seconds? If it is above the high idle rpm, it is possible that the engine was overheated and the heat damaged the ball guide if the governor assembly.
The NGK and Champion plugs listed are not the same. Physically, yes. Performance, no. Research it.
It is always best to stick with what the manufacturer recommends.
No, these plugs look typical for the most part. Overheated, if anything, not rich. Rich running would be dark black soot (unburned fuel), all over the porcelain and threads, that could be wiped off.