Just because Army would want to move salary, doesn't mean he'll be able to.
There is just no reason to believe that any scenario will make it impossible or even all that difficult to move the necessary salary. A flat cap means that Army needs to move 2 contracts to fit in a Petro extension. I've posted this before, but this is a list of the cap situations for a number of non-Central division teams:
Detroit: $34M to fill 12 roster spots
Buffalo: $33M to fill 13 roster spots
Florida: $20M to fill 10 roster spots and they are most in need of forwards/centers.
Montreal: $18M to fill 7 roster spots
Ottawa: $39M to fill 14 roster spots
Columbus: $10M to fill a couple roster spots (exact number is murky considering which injury fill ins they view as NHL players next year)
New Jersey: $25M to fill 13 roster spots
NY Islanders: $9M to fill 3 roster spots
NY Rangers: $13M to fill 7 roster spots
Philly: $11M to fill 8 roster spots (with no one but Nolan and Lindblom due real money)
Pittsburgh: $12 to fill 6 roster spots
Washington: $9.5M to fill 6 roster spots
Calgary: $16M to fill 9 roster spots
Edmonton: $10M to fill 7 roster spots with Athanasiu and their 2nd goalie being the only spots owed real money
Los Angeles: $20M to fill 7 roster spots
Vancouver: $17M to fill 8 roster spots
A flat cap reduces the value of guys like Bozak and Allen but it absolutely doesn't make them impossible to move. Every source is indicating that GMs and the league are talking about a flat cap being the worst case scenario as there isn't any desire from anyone involved to go lower: neither the owners nor the players want that, so everyone has incentive to avoid lowering the cap. However, any scenario where there is a flat cap will include some form of compliance buyout that would allow Army to get out of one of the two contracts he would need to get out of AND open up cap space around the league when 75% of the league gets rid of their big problem contract.
There just aren't realistic scenarios where moving $1-3M in bloated cap space is impossible. We aren't talking about moving guys who are overpaid by $3M each. We could also very likely manage to fit Petro in while retaining about $1M in cap space on either Bozak/Allen which makes them even easier to move.
The concern about Army being unable to shed the contracts of a couple good but non-core guys is about on par with concerns that we are going to see teams fold due to lost revenue.