A diaper will not prevent a male from getting to a female. Wearing a diaper outside is disgusting as most females urinate when they get outside, this means she will potty in the diaper.
Silent heats are very uncommon. And while there may not be blood, there will be a change in behavior of both the bitch and the dog. I would not worry about that at all. Dogs that do not live together should be supervised outside when they are together.
When the bitch is ready to be bred, the dog will be acting like a nutcase. He will not leave her nether-region alone and she will flag. It really doesn't look like play. Yes, they may cavort a little prior to standing, but it is really hard to miss when the dogs are actually taking care of business.
Call your breeder and have a discussion with him or her. They will give you advice as to when to neuter. The only AKC stuff is whether you have a limited registration or a full registration. If it is limited, puppies out of your dog cannot be registered. It does not require that you alter your dog.
If you actually signed a contract agreeing to spay/neuter, honor that. If it says by what age, honor that.
Jenna, RN CGC & Babs, CD RA CGC HIC
Heidi, RA CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear CGC