MJ. You may remember I had similar feeling about wanting Fat Bird to go broody and sit. She tried once at about 6 years old and I cant remember exactly what happened but she didn't hatch.
She tried again in my last year in Catalonia. She was laying a few eggs a year which were elongated with thin shells. She laid these in the house nestbox and did manage a few days sitting. I'm sure I wrote about it on BYC, probably on Bob's thread. It was pretty heartbreaking for me. Fat Bird broke every egg eventually coming and going from the nest box. Once all the eggs were broken she gave up being broody very quickly.
I didn't interfere. I hoped very much that she could produce even one little plump bird that would one day be Fat Bird number two.
She knew she should be the hen sitting for the tribe, especially after Ruffles's death. Fat Bird was usually the one who would drive younger tribe members off the nest if they looked like they were trying to sit. I am likely to encounter a similar problem with the field hens at some point and I can't let Fret sit due to her strained leg.
There isn't a right answer here and like myself with Fat Bird you've gone beyond just what a broody hen wants and what you want seems to have equal wieght in the matter now.
I never candle eggs. There isn't any point with broody hens. I've never had an egg explode under a broody hen and any that have been broken I've done a nest strip and clean up. I can't count the number of times I've read abput people candling eggs where they dropped one or two, transfered bacteria from the handling to the eggs, shocked the egg in the last few days, possibly turning the chick in the egg when it needed to be stationary.
Knowing whether an egg is fertile or not doesn't mean one knows it will hatch under a broody hen. If you recal when Fret first sat I imported some fertile CCL eggs. They were fertile but Fret left them at the nest when Dig and Mow had hatched.

The nest makes an enormous difference. If the nest is poor, there are lots of things that can make it so, then this affects the hatch rate and even the willingness for the hen to sit the full time. Nest visitors are always a problem be they other hens, roosters and/or pests.
You should know how keen I am to have hens hatch their eggs and you will also know that apart from making sure the hen gets off the nest to eat etc I let the hen have control and if she, or the eggs quit then I come behind and clean up.
Most people find it incredibly difficult not to try and help/influence a broody hen. I can't think of a single instances where I have interfered and made a positive difference.
Nobody can give you the right advice. This is between you and Mary.