Now you have more of an understanding why so much DIY conversation revolves around aging (aka. "steeping").
Having the patience to let a mixture become what it wants to become, is the least costly... and most difficult, part of DIY to master.
A frothy appearance is fairly common when using a hand mixer... especially if a milk frother is being used. You should see it dissipate pretty quickly though (half an hour; or so?).
Over-mixing can indeed cause flavor loss in some flavor concentrates (some more than others). But, as you are already learning, it is important not to judge a recipe only on first impressions. Some recipes can be good right after mixing. Others benefit from time. vaping a little, at various intervals, is the only way I can judge when a recipe is ready.
As for the chemical taste/smell; this can often overpower the desired flavors in the beginning. Don't panic; these often dissipate with time. Sometimes it can be helped along by letting the mixture "breathe" (breathing is leaving the bottle completely uncapped (no cap; no dripper tip)). This allows the chemical notes to evaporate.
However; breathing is a double-edged sword. It also makes it easier for wanted flavor notes to escape and weaken the flavor. If you want to try breathing; I would encourage you to divide the batch, and breathe only a portion of it, to see if it is a help or a hindrance. You can easily compare the uncapped sample, to the capped sample, and see which you like more.
If you are really concerned that the hand mixer is causing problems; try mixing two identical mixes. whisk one with the hand mixer. Just cap and hand shake the other. See how they compare to each other fresh, and then a various intervals, as you let them age. If they either start out and stay the same, or start out differently but become the same, you are not having any adverse effects from the hand-mixer. If they end up tasting different; you are likely over-mixing.
