Hi Heather,
As I'm sure you know, Guelder Rose isn't a rose at all, but a Viburnum (Viburnum opulus). As far as I can see, the advice for Guelder Rose would be the same as our advice for other Viburnum species -- and that is probably not to feed, but definitely don't allow the tortoise to eat the berries. Here is our entry for Viburnum and I think there might even be a photo of Guelder Rose there.
https://www.thetortoisetable.org.uk/pla ... 13eibgnaM8
Re Hawthorn, the seeds are definitely toxic and contain amygdalin, which bonds with sugar and then turns to hydrogen cyanide in the small intestine. I found this on one site:
"Amygdalin decomposes into three parts. Hydrogen cyanide, glucose and benzaldehyde. Benzaldehyde, which tastes strongly of almonds, amaretto or marzipan is a common chemical in many young plants. The flowers of hawthorn, sloe and cherry all smell of benzaldehyde."
As far as I can see, benzaldehyde is not toxic, except in very large doses, so theoretically the flowers should be OK, but I don't know if they actually contain hydrogen cyanide as well as benzaldehyde.
I did find a site that said "The Folklore Society of Britain found that hawthorn blossom is the most unlucky. If brought into the house it would result in death." LOL, but that is just folklore. Still, I think I would err on the side of caution and not deliberately feed any part of the plant, just in case.
Nina