I've heard that Kadri would have re-signed for 6.5x7 or 6x8. Take that with a grain of salt though.
There are always options out there to solve 2C and middle pairing defensemen needs. It isn't this black and white of just Kadri or just Manson/Newhook. Kadri could have been re-signed. Could have made a sign and trade for Horvat. Could have gone for a rental like ROR. Could have gone risky with a 1 year signing like Domi/Strome. Could have sold Byram. They could have dumped EJ and signed Kadri along with still having Newhook and Manson. There is so much grey here that simply saying it had to be this or that is framing the debate unfairly.
To me, there were two paths that I would have investigated heavily. First would have been to put Byram on the market and tried to find a long-term 2C solution. A kid like Byram suddenly loosens a lot of names, and while I wouldn't have traded him for Dach (as I'm not a huge Dach at center fan)... that's a name that would have been a viable option. IMO there was (and still is) an option to move Bryam for a younger player in than 2C with legit 1C upside. That sort of move to solve the issue for 5-7 years would have been my first option.
I think the other realistic is simply choosing Kadri over Manson. Kadri was certainly going to regress and the contract would end poorly. Zero question about that. It would have certain window shutting moment at somepoint in the future. You also know he fit like a glove here and was still playing at a high level. If you got 3 years out of him at a 2C level, you secured 3 years of a potential to contend. Last year certainly would have been better... and I'd say that having Kadri over RyJo for the 2 seasons would be a lot more comforting. Certainly not without risk, but this would have been a known risk. The Avs were in the negotiations until Klingberg signed in Anaheim. They just were not willing to pay the price.
On Manson... I simply wouldn't have signed him or anybody for that role. Let the top 4 group of Makar, Toews, Byram, and G just figure it out with one of the lefties on their offside. Sign some cheap RD for depth. If/when it proved to be an issue, simply pay a 2nd for a RD rental. They are available every deadline and that cost is worth it to not get tied into a bad contract.
Ok, we're getting somewhere -- reading between the lines here, you are saying you would have kept Kadri at this fictional 6-6.5m rate and not re-signed Manson. I don't agree he would have taken that large a discount but obviously, that's all speculation. let's go with your assumption.
Avs were with ~700k of the cap at year start due to their apparent moral objections to LTIR. to add Kadri at ~6.5m, we must cut further -- and it's obvious who wouldn't have been available to us, the last second signing of Erod at 2m.
So it's + Kadri and - Manson / Erod, and the opening night roster would be something like, iirc:
Lehk - Mack - Rantanen
Nuke - Kadri - Newhook
Cogs - Compher - LOC
AHL - Myers - Hunt
Toews - Makar
Byram - G
Hunt - EJ
Boy I dunno man. That's a pretty non-physical/smallish D corps I think we all would have been dreading fielding in the playoffs. Byram - G is especially scary. The top 6 is marginally better but we're still counting on Newhook in the top 6 and the bottom 6 is still garbage. imo, this roster is about the same level as the one we went with.
Now let's get to the other various ideas you tossed out.
STL almost certainly wouldn't have traded ROR here, in division. ROR very likely would have thrown a fit and torpedoed a deal anyways, probably making the Avs unlikely to try it, the trade price he eventually did get was ridiculous, absolutely astronomical, and he was still a 7.5m cap hit. I don't see that as an option on any level.
EJ said he wasn't going to waive his NTC, and actually exercised it, didn't he?
You wouldn't have traded Byram for Dach, yeah I agree, especially at the time. Who else do you think would accept Byram for this hypothetical young 1C, then or now (probably far more likely now, after a year of relative health)?
Who were these RDs we could have traded for at the deadline?
I still don't see obviously better alternatives anywhere here, and that's my point. It's easy to say stuff like "We should have kept Kadri" or "Just trade EJ duhhh" but the reality is that the other options had undiscussed MAJOR downside or simply weren't on the table.