In the UK they will only show heavy metal and fluoride contamination which is the same by the looks of it. Most good nutrients should have enough but since I've been using LED I have definitely used more. Also pH has a big part to play in availability for the ladies.
I would still shop there mate
Unfortunately all soil bags have breathing holes and whether the gnats were in the soil from manufacturing or entered at the store it's a problem that you are going to stumble upon here and there. I have had fungus gnats a few times using different soils from different stores and also they have entered the tent and made themselves at home. Fungus gnats only eat dead root material so they are not going to be a big issue just an annoyance especially in flower when they stick to the trichs