Bonjour JPIXI! I am so glad you are now getting the wonderful peaceful feeling of watching the results of your efforts come to bloom! It is a wonderful feeling isn't it - just to feel "I did this - amazing!"
I am also glad to hear that you finally did find a source for your epsom salts - that was sure a treasure hunt wasn't it?
In answer to your questions:
1) I mix my epsom salts 2 cups to 16 cups alfalfa to 30 gallons water. Each plant gets about a gallon. I do this probably about every three weeks or so.
2) You should not be feeding your roses any fertilizers of any type, organic or otherwise about 4 to 6 weeks before frost. This gives the roses time to wind down and stop producing new growth that will just get damaged in the winter. Of course, if fall and winter there is anything like it is here guessing when the frost will come is a little hit or miss, so use your best guess. I usually stop fertilizing around August 15.
3) Certainly keep adding composted material around your roses during the cold season as you are building up the quality of the soil. Make sure it is composted first though, or you will be depleting your soil of nitrogen. Before I cover my roses for the winter, I always mound mine with a big pile of good compost.
4) Many plants are suitable for the use of Epsom Salts. Chemically, Epsom salts is hydrated magnesium sulfate (about 10 percent magnesium and 13 percent sulfur). Magnesium is necessary for seed germination and production of chlorophyll, fruit, and nuts. Magnesium strengthens cell walls and improves plants' ability to use nitrogen, phosphorus, and sulfur.
Sulfur, is necessary for production of vitamins, amino acids (therefore protein), and enzymes. It's also what gives vegetables such as broccoli and onions their flavor.
However, plants may not show the effects of a magnesium deficiency until it's severe. Some common deficiency symptoms are yellowing of the leaves between the veins, leaf curling, stunted growth, and lack of sweetness in the fruit.
Magnesium tends to be lacking in old, weathered soils with low pH. Soils with a high pH and are high in calcium and potassium also generally have low magnesium levels. Calcium and potassium compete with magnesium for use by plant roots, and magnesium usually loses. Sometimes, a soil test will show adequate magnesium levels, yet a plant grown there is still deficient because of that competition.
So, after that whole long speel, yes, epsom salts can be beneficial to other plants as well. I use the Alfalfa/epsom salt tea on my clematis, several shrubs, lilies, tomatoes, peppers, cucumbers, as well as my roses!
Keep enjoying your roses! Let me know what winter is like over there, and I can give you some pointers on winter care if you would like!!