Yes, bookings are in a similar vein to be sure. The way Glasto works tho (or at least used to beyond Pyramid and Other, maybe Holts too), is that different 'families' run the different stages, so they'd have to line up or come to some sort of collaborative arrangement.
I don't think the American shows mean anything for over here, they were massively bigger there than they ever were here.
In that case then if they do play Download I expect them to sell their day out easily and they couldn't do that in 2011 when they reunited.
Yeah I think it's been shown many times that whatever happens elsewhere isn't necessarily the case in other countries. SOAD were always massively bigger in the states and could headline multiple festivals whereas there was only one here longterm another around for a few years. The other option they played they didn't headline.