2400 propellent is NOT for mid-level 357MAG loads,
Starting level loads for 2400 are mid level load in .357. 2400 isn't good for bunny phart target wadcutters, and isn't the most efficient powder for mid level loads, but it works well enough, and gets better as you increase the load, until you get to full power.
It is much more tolerant of being loaded lighter than full power, than WW296 is.
The outside of an old powder can could be an indicator of the inside, but may not be. I've seen lots of old cans with some or even massive surface rust on the outside, the inside still pristine.
IF there is no sign or smell of degradation, load a couple rounds, with bottom end data (or slightly less) and test fire. Assume NO published data matches the burn rate of what you have, and work carefully, testing each increment until you get where you want, or start having pressure signs.
I won't say there is NO risk, and of course, the safest thing to do is dispose of it, but if you want to use it, based on what you said, I think it will work.
Might not work exactly the way you expect, so be careful.