Rose petals have a perfumy flavour to them, which can be nice sometimes but unpleasant others. They are perfectly edible, though. My recommendation would be to try some rose petal syrup, some turkish delight, rosebud tea and finally some fresh or candied rose petals to give yourself a rounded view of the flavour.
Rosehips make a DELICIOUS candy - but they cannot be eaten fresh. They start out really, really sour and are full of itchy little hairy seeds. They need to be gutted and cooked in a thick sugar syrup to be palatable, although they can be used just dried whole in herbal teas.
I wouldn't know about the stem. I imagine the stem on a mature plant would be tough and stringy, but possibly new baby shoots could be eaten? I know you can eat the baby shoots of blackberry plants. Slice them horizontally and dry them, and you get black-purple, tiny star-shaped candies that taste EXACTLY like blackberry flavour sweets.
Lavender is also edible, and very tasty. You can grind the flowers up and add them to sugar to make flavoured baked goods, or dip the whole flowers in beaten egg white and sugar and dry them.