The Blues taking a forward is definitely the safe bet.
The path to taking a defenseman looks really narrow to me and the board would have to break just right for it to happen imo.
Lambos has medical issues that we still don't have info on but let's say the prognosis is good for his health in the future. At that point it's very likely he's gone by 15 leaving us with Ceulemans as the best D option. No other D is ranked in this range around 16 by most scouting services so unless the Blues really like Ceulemans that leaves a forward and there are plenty to choose from in that range which is why I suggest it's the safest bet.
I still think we're taking Cossa but it wouldn't surprise me to see a forward taken instead.
It's hard to predict, we don't know if teams prioritize a specific position. Every year there are also a couple of teams that really like a specific player and are willing to draft that player higher then they are projected (ex. Moritz Seider). You could be right, on the other hand no one expected Chychrun to still be on the board when Arizona picked him at 16 in 2016 because other teams had other priorities. Same goes for Barzal in 2015 (what was Boston thinking?!).
Who knows what strategy the teams are going to deploy. If we're specifically looking at defenseman almost all scouting agencies are expecting Power, Clarke, Hughes and Edvinsson to go top 10. After that it's a crapshoot. Lambos is ranked from 3rd to not even being drafted in the first round at all. Ceulemans is mostly ranked between 15 and 31, however because this draft hardly has any high ranked right defensemen (only Brandt?) I wouldn't be surprised at all if a team will reach for him.
Lets summarize (IMO):
Potential top 15 defensemen:
Power, Clarke, Hughes Edvinsson,
Ceulemans, Lambos
Potential top 15 forwards:
Beniers, Guenther, Eklund, McTavish, Lysell, Johnson,
Sillinger, Svechkov, Coronato, Lucius, Olausson, Raty
Potential top 15 goalies:
Wallstedt,
Cossa
Any of the bolded players could be in our range.
Not a fan of Raty though.
EDIT:
We also don't know if the teams will draft differently due to not been able to heavily scout a lot of the North American-prospects. Due to this we could see a boost to the stock of some European players, for example, catapulting Svechkov to the top 10. Also we don't know how much issue teams are going to take into potential injury problems (Lambos, Lucius, Wallstedt). If the former is true, I wouldn't even be surprised if teams are relunctant to draft Wallstedt (apparantly he has back problems and had multiple minor concussions) and pick Cossa over Wallstedt.