Hi Hel
I can't help you regarding seasons in an older bitch but can tell you what it's like having a dog and bitch in the home during a season - it's a complete nightmare, I had just about the most stressful 4 or 5 days I've ever had in my life.
My original plan was for Jimmy to go to my sister during Misty's first season but unfortunately circumstances changed and this wasn't going to be possible so I booked him in to be neutered. Misty chose this very day to come into season meaning the effects of the op hadn't had time to take effect. I live in an open plan flat so had to build a barricade to keep them apart, despite this they both still made attempts to get to each other. Misty would waft her backside at Jimmy from the other side of the barricade driving him mental. ( On Champdogs I read a story of a dog impregnating a bitch through the bars of a crate.
) One day I took Jimmy to the park across the road and as soon as I let him off lead he made a run for it back home, dashing across a main road in London rush hour traffic to get to her.
At night I had to sleep with Misty on the bed, her puppy pen surrounding us with also another barricade to try to prevent Jimmy getting to the bedroom area - he managed to get round the first line of defence every night but never past the puppy pen luckily.
Like Fletch, Jimmy had never shown that much interest in bitches in season prior to this - he'd have a good sniff but wasn't obsessively persistent in the way I've seen some other dogs behave. Being in such close proximity to Misty was a completely different matter though.
I'd definitely ensure that plans are in place for Fletch to stay at your parents' house during the season. Apologies for the mammoth essay but hopefully it will be of some help in your considerations.
