I said if I'm gonna tighten it now and use the first hook itself, what will happen when after months of use it becomes loose? Which hook to use then?

Exactly :)

Straps should be adjusted to whatever length is comfortable. If you're finding straps are constantly falling down and/or digging in painfully, that's often a sign the band size is too big, as strap length has a level of correlation with band length. That said, strap length can vary from brand to brand, and some straps allow for a greater variety in length than others, so sometimes if people have an especially long torso or especially short torso or highset breasts or lowset breasts they can run into problems with not being able to shorten straps enough or extend them enough even in the right band size. I would advise fiddling with the straps a bit whenever you get a new bra to see if you can find a better position than it's already in. And make sure to re-check the straps every now and then - the elastic in the straps wears out over time just like in the band and the straps adjusters can get moved slightly from wear and from washing in a machine, so readjustment is occassionally helpful.

Band riding up in back implies the band is too large. Occasionally too large bands can also ride down in back, but it's less common. The ribcage is wider towards your shoulders than towards your bellybutton, so the circumference around your underbust is smaller than the circumference 1 inch higher, 2 inches higher, etc. Additionally, a band sitting level is going to cover less area than a band not sitting level. So a band is only going to be able to ride up if it stretches to larger than your underbust measurement (a well-fitting band stretches to approx. your underbust measurement).