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To get the best fertilizer for your roses, you need to start with a soil test to see how your soil is currently doing. The Garden Professors blog from 1/13/18 (Facebook.com) mentions that fertilizing without testing can easily lead to toxic high levels of nutrients. When nutrients are way out of balance, it also affects the ability to utilize calcium, for instance. On the whole, you are right that recommendations are all over the map which only indicates that there isn't a one-size-fits-all answer. Some sites recommend rotating whatever food you are using to balance things out. It sounds like a lot of guesswork and not much science are behind commercial products. 20-20-20 sounds mighty strong unless you are using it sparingly.

No, hybrid anything doesn't come true from seed. Another issue is the rootstock. Many, not all, roses are grafted onto sturdier rootstock to help the plant perform well. You can try taking a cutting for same color but you still have the issue of rootstock. If you dig into the exact name of your rose, you should be able to find whether it is grafted or grows on its own roots.

Perhaps a Flava Trumpet Vine, Potato Vine, Bougainvillea Vine or a Jasmine Vine would be good choices. The roses would not likely do well with the wind and would need more sunlight.

This depends on weather and how fast-draining your soil is. Sandy soil does not hold water well; therefore, it will need more frequent watering. Mulch slows water loss from the soil and helps keep roots from over-heating. Hot or windy weather dries plants out faster. The general rule is an inch of water each week from the time roses break dormancy through fall. If rainfall is adequate, there is no need for supplemental water with one caveat; if the rain is a sudden cloudburst, much of it runs off instead of soaking in. To improve sandy soil's water retention, add 2-3 inches of compost as a top-dressing annually.

I'm not sure what your question is. If it is failure to rebloom, then one questions whether they are getting enough sun (6 hours minimum), enough water, or whether they are over-fertilized, producing green growth at the expense of flowers. I assume you have vigilantly checked for small insects like aphids, spider mites and white flies where flower buds form. You say you are spraying - is this for black spot? What are you using? Certainly an infestation/infection will reduce bloom. Watering deeply and at soil level is recommended. Watering leaves twice daily is a set-up for leaf diseases of all sorts. Do you perform annual pruning to "open up" the rose to sunlight and to spur young growth. Canes lose vigor as they age and bloom less than younger wood. Finally, are you sure these are reblooming roses? Some old garden roses and climbers do not. If it seems that all the above check out as OK, then get a soil test. It is possible to have toxic levels of nutrients as well as severe deficiencies of micronutrients.