Did you seriously try that smoke test by yourself???????
Here is a little science for you…
While doing a smoke test, it is required that a minimum of positive pressure be maintained into the system to allow the smoke to escape from any possible leaking area.( keep that bit of information in mind )
Now you start pumping up your homemade smoke machine to begin filling the engine intake ( BTW, did you remember to fully open the throttle body butterfly during the testing?)
Now what happens to the positive pressure your applying while pumping when you stop pumping to begin inspecting for any possible leaks in the engine compartment, under the vehicle, back around under the fuel tank and by the filler neck?
Concerning your spark plugs ….
If NGK’s worked and were the correct fitment for your engine.
A new set of replacements should cause any issue(s) UNLESS…
They were improperly gaped
Or if they were counterfeit NGK plugs. Yes that is a thing these days, you can look up counterfeit NGK spark plugs.
Mixing grades of gas, though not a great thing to do should not cause any major running issues.