I must say Magnum has me stumped. I'm an electrical contractor specializing in solar and off grid power systems. However, I've never worked with that particular Magnum inverter. I've never heard the term "branch-rated", it's not in the code book so I don't know where they got that from. I assume they mean that you shouldn't run your branch circuits from the 60 amp built in breaker and need to use other smaller breakers that are appropriately rated for your branch circuits. That would mean installing a normal breaker panel on the inverter output and putting your single pole 15 and 20 amp breakers in it.
As for putting a breaker in neutral conductor, that is pretty much never done.
In fact, the NEC doesn't allow it unless the breaker is designed to open both the hot and neutral poles simultaneously and there's no way they could be operated independently. A standard double pole breaker would meet this requirement. However, I can't imagine why they would want you to wire it up that way.
As several other posters have noted, the breaker supplying the inverter will limit the current it can pass thru. In the RV world this will be no more than 50 amps no matter what. Since the inverter is rated for 60 amps I don't see a problem there.
If you're curious enough I'd call Magnum's technical support department. From your call I would bet they will learn that whoever wrote their manual did a poor job.
I would just install your sub panel on the output and put the normal single pole breakers in.
