Hi and welcome to the forum.
I think you’ve raised valid concerns like possibly already diseased plant, possible contaminants/fungal spores, overwatering a stressed plant, and full sun.
Whether there had been existing fungal issues, unless you want to try to return the plant, let’s see what we can do.
First, potting mix —
- What was the composition of the potting mix/soil you used?
- What made you conclude/how was the plant “overwatered”?
- Does the pot drain properly?
FULL SUN —
You never want to put newly purchased plants in full sun. They need to acclimate to intensity and duration of direct sunlight, especially if they had been in protected display areas.
Peppers in general tend to not need full sun. Especially stressed plant should be protected from sun exposure, extreme heat, and water stress experienced by container plants in full direct sunlight. Try
- protecting from mid-day sun (overhead shade, shade cloth, etc.)
or
- provide just morning sun from sun up to no later than noon or 1pm
or
- put in dappled shade of a tree
FUNGICIDE —
I looked it up and active ingredients are essential oils with glycerin as emulsifier. This means treated plant is susceptible to sunburn and may also be to high temperatures. You may need to keep even more protected until it shows healthy new leaves.
I would snip off and discard significantly damaged and yellowed leaves like some that are on the photos.
You can clip off the bottom most 2 or three (or more) branches altogether. They just divert water and nutrients. there should be a strong V branch that is naturally growing that firms the plant into Y shape. All lower side branches are unnecessary.
I started a regimen this year to just keep the leaves if healthy and cut or pull at the sucker shoots side ways to break off.