As the inventor of the orchid-growing technique, let me take a shot at these:
1) I recommend against slow-release fertilizers. They are typically little spherical particles that can help trap water between media particles, helping suffocate roots, and they will ultimately settle into the reservoir, making the dissolved fertilizer concentration too high. Very dilute, water-soluble fertilizers are a much better choice.
2) Anything inorganic can be used, but LECA is by far the best. I have experimented with many, many materials, and have yet to find anything better.
3) The best time for any repot is just when new root growth is emerging from the base of the plant. When roots grow, they "tailor" themselves on a cellular level to be optimal for the conditions into which they are growing; once they have grown, they cannot change. Move a plant into a different environment, and existing roots are likely sub-optimal, and will eventually succumb. The new roots, optimized for the new environment, will sustain the plant. The rate at which the old roots fail depends upon how different the two environments are.
4) Any plant can be grown in semi-hydroponics, but here's the thing: "Semi-hydroponics" does not completely define "orchid culture". S/H primarily addresses water and nutrient delivery, and does not address any of the other parameters of orchid culture. Your success or failure depends upon how well the constantly moist, airy medium "plays with" all of your other parameters, and meets the requirements of the plant. And since plants have individual needs....
I'll give you an example. I find that paphs are essentially a "sure thing" in S/H, while others kill them. Some folks view hard-cane dendrobiums as great "S/H beginner" plants, but I - with more experience with the technique than any other person - cannot grow them that way to save my life. The difference isn't the pot design or medium, it's the rest of the conditions.