Gaura lindheimeri.

Happy to share a suggestion. The leaves on yours look more like D. stramonium to me. (And after 2 hours, a wrong suggestion usually has objections.) But for either species, agree, likely not included intentionally, or at all, with your wild seed mix. They just show up sometimes. https://gobotany.newenglandwild.org/species/datura/inoxia/ https://gobotany.newenglandwild.org/species/datura/stramonium/

you can cut the leaves. remove rotting part, let it dry in room temp 3-4 days, then plant it, (S. trifasciata and all Sans that I know of can be propagated by leaves cutting, rearrange leaves in pots as you like. water sparingly, only when medium dried. Room temp bright light but nut direct sun for now. treat like succulent. avoid too much watering. most likely you will end up success.

A 5 stage 100 gallon per day reverse osmosis filtration system can be had for under $300 and you can install it (easily) yourself. An ionic exchange water softening system won't work because it exchanges Na and Cl ions for the minerals that make your water hard, Ca, Mg, Fe, etc. Hard water causes a build-up of (bi)carbonates in the grow medium and can increase media pH over time to the degree that many essential nutrients become unavailable for uptake. It would be best if you were fertilizing regularly with an acid-forming fertilizer, and flushing the grow medium each time you water, such that the entire soil column gets moistened and at least 20% of the total volume of water applied exits the drain, carrying any build-up of carbonates and other minerals nutrients with it. My suggestion would be to read this short piece I wrote about Good Growing Practices, and compare the information to whatever regimen you have established, to see where improvements might be made. You can also start monitoring moisture levels by using a 'tell' to 'tell' you when it's time to water. The more water your grow medium holds, the more critical getting your watering intervals spaced appropriately becomes . You can depot the plant and examine the roots. If roots are scraggly or dead, or they smell sour and unlike what fresh soil smells like, you're likely over-watering. If the damage is mostly in older leaves, it might be a fluoride toxicity as fluoride accumulates in tissue and can be eliminated from the plant only when affected leaves with the accumulation are shed or removed. Should you start using a water supply w/o fluoride, the new growth will show no symptoms of a fluoride toxicity, but affected leaves would continue to show damage until the affected leaves are shed or removed. Al