I’ve a sweet tooth and like a bit of honey in my tea. So, I put a dab of honey in the cup while the kettle goes, then pour the boiling water in, and use one of those little electric milk frothers to stir it up thoroughly. While the water is still spinning, I toss the tea bag in, and throw the lid on the cup to let it steep for however long.
Water foaming when heated is caused by impurities in the water rather than the heating mechanism. So whatever is causing your water to foam is in it either way
It would only be able to heat the tea to above 100C if that point of zero nucleation remained undisturbed, and the tea held directly within it; in other words, it’s not possible.
Assuming there’s an amount of water within the cup that is above 100C, while the rest is at or below 100C. As soon as you grab the cup out of the microwave, the water will start to slosh around in the cup, equalizing the temperature. If there are any major hot spots remaining, they will be well below 100C, and almost completely equalized when the tea bag is added.
And we all know what science says: if you can’t see something, it isn’t there!