Nisky is trash at this point, maybe a fine #4 dman but he has struggled. Flames probably dont have interest, and jaskin and a 4th doesnt make them eager to move their first.
Overpay for Duchene, they'd probably want a bura back in the deal as a roster player. Maybe: Johanson, 2019 1st (WSH), Bura for Duchene and if he re-signs the sens get the 2020 2nd

I’d rather see Panarin on the top line with Ovi and Kuz, but not likely

These trades probably make the Caps worse. That D unit is sub-par at best.

Getting rid of Niskanen is addition by subtraction. He’s been bad. Siegenthaler has been great.

The entire D core has been bad lately, top to bottom. Niskanen has been no better or worse in his own end than Carlson, Kempny, or Orlov. Trading Niskanen forces Djoos into a lot more ice time which he's not ready for, and forces Bowey into playing every day which he's not ready for, at least not at a level that could get the Caps back to another cup.

While I agree Siegenthaler has played very well in his limited time and should be back up with the Caps you're not going to ship out a reliable defenseman and ride it out with Djoos, Bowey, and Siegenthaler who simply aren't ready to be thrown into that kind of role.