So the plan would be to carry on thru out the entire season with a 20 man roster??

that's what they are already doing

Yes that is the plan.

It doesnt make sense to trade for both soucy and chychrun. Chychrun would play the left side next to Bouchard.

Edmonton has got to find a way to trade for a dman without trading Broberg. He is the last D-man that could be a number 1 that Edmonton will have before rebuilding.

I disagree. I think they should be using Broberg and a 1rst to trade for a legit top pairing dman at this point. Not saying Chychrun is the guy they should go after given his recent injuries. But I dont think they should be waiting around anymore HOPING Broberg turns into a top 4 dman. But say they do get Chych:
Nurse-Ceci
Chych-Bouch
Niemo-Kulak
Murray

Would that be significantly better than what they have now??

It would be better short term. The reality is that there will be no more established defencemen available for even the next 3 years. Teams do not trade top 4 dmen. If you want a defencemen you have to draft them. Broberg is the last chance at a solid player playing 1-2LD in edmonton and staying for any amount of time. Chychrun has injury issues and only plays about 60 games a year, he is Klefbom 2.0 and very likely to exit the same way, if he doesn't he is gone after his contract.