Since it says the minimum circuit ampacity is 25.2, you need a cable that will handle at least 25.2 amps. For this, #10 gauge is what you need so you're in fine shape there.
Since it says that the maximum fuse or breaker size is 40 amps, then you may install either a 30 or 40 amp breaker. Normally, you'd only use a 30 amp breaker max for #10 gauge wire. Air conditioning equipment is one of the special exceptions in the code. Since you already have a 30 amp breaker installed, the a/c may run just fine. If you get some "nuisance tripping" of that 30 amp breaker, I would install a 40 amp breaker in the panel for sure trouble-free operation. OR... you can just swap out the 30 amp breaker for a 40 amp breaker right away. Your choice... no special hazard either way.
No, the disconnect does not need to be fused. You may use a fused disconnect if you want to, but this would be redundant, since the overcurrent protection will be provided by the breaker in the panel. You can just use an ordinary weatherproof (nonfused) disconnect. These are really cheap (compared with the fused variety) and come in 30, 60, and 100 amp sizes. You may use a 30 amp size, but the 60 amp size is often more readily available and will be fine. Just a little more heavy duty.
"Nothing is foolproof to a sufficiently talented fool"