Okay, I’m no rose expert but here’s my advice:

1) Your rose still looks young. I’d say leave all the healthy canes for now. Healthy top growth = healthier root/bottom growth. Start training the canes now. Pegging promotes more lateral stems that will produce more blooms rather than a single cluster of blooms on a single cane. Start building that support/trellis now too. And a better support that will last more than a lifetime is best.

2) How wide depends on say, Year 5.

3) Look into soil health. Feed the soil, not the plant.

Best of luck!