You should use Motorex SAE 7.5 for the damper and Motorex Supergliss 100K for lubrication.

Mucky Nutz have a reverse-arch-style fender: https://muckynutz.com/MTB/face_fender_reverse

First you could try if you can put in the valve with a rotary movement, like screwing the valve in. If this also does not work, you can check, if the tubeless tape was pushed into the valve hole. If so, just cut the tape out of the valve hole and try it again.

As long as the tubeless valve seals, you don`t have to replace it.

You can find the DT article number here: https://www.dtswiss.com/en/support/conversion-charts/end-cap-solution-mtb-rear/

I would only use original DT spare parts. DT Swiss only offers a sleeve to switch from 15 mm to QR.