I've just tried to catch up with all the posts so apologies if this has been posted elsewhere.
The Vadic being revealed to be a changeling was really poor and let the episode down IMHO.
We've never seen them needing to cut themselves to detach a part, add in the smoking and these red herrings go further and really start to contradict what we know about changelings, maybe it is just to throw us off the scent, but in the same way as if we start seeing Borg being able to fly, it just feels forced when better writing could still keep things up in the air.
I am also not even sure what the point of cutting part of her hand off is supposed to signify, maybe there is a pay off later, but this is another symptom of the streaming service adding in elements that picked up later and can make watching some episodes difficult as they make no sense first time around, not an issue with subtle plot threads, but this made little to no sense and a little exposition would have improved matters a lot.
The whole episode was good as the plot got advanced, although it seems Vadic is playing her own game not doing as she is told and crippling the Titan and then being ordered to destroy them, with the changeling on board as well, what purpose does a two pronged attack have, unless the changeling on board is trying to achieve some other agenda, they could simply destroy the ship a dozen different ways if they wanted to.
I've read some posts about Jack being part Borg through Picard and his swimmers having nanoprobes in then......the visions are the collective calling to him, this could explain why he sees Seven in his vision as her nanoprobes or physiology may hold the key to unlocking what he sees.
Overall a good strong episode that concludes the nebula arc quite well but still leaves us with questions as to why a lot is going on. My personal theory is there are a lot of callbacks to The Enterprise C, what with The Red Lady and models of the C being shown, I strongly believe that something about the battle of Narendra III will be an action that comes back to haunt Picard, ironic considering he has no memory of it as it is from a collapsed timeline. I also think there may be some link between the changelings needing to shed another skin of evil and creating a new Armus, both giving the potential to the Tasha Yar reference in the show.