Yeah, that's a main lug panel, usually used for sub-panels. So your 'service disconnect' is technically the breakers outside, so you can get away with this.
I don't think locking it is an issue. I'm pretty sure that the FD has bolt cutters. Also, they should be able to just break the seal on your meter channel and pull the meter to kill power to your house, if need be (residential power runs through the meter; commercial and industrial services are usually direct connects and pulling the meter will not disconnect power)
RE: Challenger breakers in outdoor combo service: I think it's hard to say for certain w/o seeing inside of it. Some are made with a separate busbar for the breakers to plug into; in that case you could just replace them yourself (though the busbar would be hot). Other enclosures have no busbar, and the have lugs straight to the breakers or some other similar arrangement. My guess is that you have a small busbar for those 2 breakers, but I would personally pull the cover to confirm.
(this is assuming that there's 2 different covers/compartments for meter/breaker. Some older ones shared the space and you'd have to open up a single cover to get in there)
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