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It is not listed as a toxic plant, but it is not an edible plant.
Always contact your vet if your animals consume something that they should not.

It sounds like your plant has a Rust Fungus. This can be due to damp growing conditions and poor air circulation.
https://www.gardeningknowhow.com/plant-problems/disease/learn-about-plant-rust-disease-and-rust-treatment.htm
Neem Oil can be used to treat this fungus.
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

Your Mandevilla plant can be overwintered with some care.
Here is a link that will help you.
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/winterizing-mandevillas.htm

Mandevilla Vines are hardy in zones 9 - 11.

I know as gardeners, we like to try to push the growing zones, but in the case of a Mandevilla Vine you will not be able to winter over this plant
in zone 8.

https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm

No, never fertilize a plant that is going into dormancy.

This article has information on wintering over your Mandevilla.

https://www.gardeningknowhow.com/ornamental/vines/mandevilla/winterizing-mandevillas.htm

This is most likely a disease called powdery mildew, which is caused by a fungus. These articles discuss how to treat it:

https://www.gardeningknowhow.com/plant-problems/disease/get-the-cure-for-powdery-mildew.htm
https://www.gardeningknowhow.com/plant-problems/disease/powdery-mildew-homemade-and-organic-remedies.htm
https://www.gardeningknowhow.com/houseplants/hpgen/powdery-mildew-treatment.htm

Mandevilla can suffer yellowing or browning leaves from a few different causes.
They need partial sunlight for at least 6 hours a day.
Over or underwatering can cause yellowing or dying leaves. Make sure the soil is moist but not soggy.
Insects can cause leaf damage and yellowing leaves, inspect the underside of leaves for signs of insects and treat with a insecticide if needed.

I do believe your plant may be suffering from a leaf spot infection. These often start with a brown or yellowing spot on the leaf, which spreads from leaf to leaf.
This infection can also spread to other plants in your garden.
Prune off any diseased leaves from the plant and remove any that have fallen unto the ground under the plant.
Treating the plant with a fungicide that is made for blooming plants may help but it is possible the disease is bacterial and will not respond to treatment and should be removed from the garden.

https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm