Corbyn said he divorced his wife because she insisted on a grammar for their child and please correct me if I am wrong - their child did end up going to one didn't they!
I don't think Corbyn has ever said this directly. His wife said that her wish to send their child to a grammar contributed to their divorce. Presumably after the divorce she had custody and sent their child to the school of her choice. Unless he was prepared to murder his wife to gain custody himself I'm not exactly sure what you think he should have done to prevent this.
Whilst this point has been done to death opponents of grammar schools are not opposed to grammar schools themselves as being the source of all evil, they are opposed to a selective SYSTEM. Whether your child attends a grammar or a secondary modern they are still part of that system. You may as well argue that parents in a selective area are hypocrites for sending their child to a secondary modern if they are opposed to selection.
There's also plenty of posts on Mumsnet from people looking for an area of the country where their child can have a stab at the 11+ but still go to a good comprehensive if they fail. So you live somewhere like Thame in Oxfordshire to access the selective system that means most children in Bucks have to go to secondary moderns but you yourself are avoiding any bad consequences for your own children by positioning yourself in comprehensive Oxfordshire!! I don't blame parents for doing this - clearly exploiting county borders in this way is a perfectly sensible thing to do on an individual level. It just slightly annoys me that doing this never seems to raise any ethical issues for people, especially when people in Kent and Bucks who can't access comprehensives get criticised for not sending their kids to them.
Personally, I've always said I would like a law that says before you sit the 11+ you have to sign a paper saying if your child fails you have to send them to the nearest secondary modern. No hopping over a county border, no going private. If you support the selective system, you support the system fully and you go to the school that the system says is "right" for your child. It will never happen.