1. In my city, it's better to give a wider range of preferences (as long as it's truly something you like) given the huge demand for quota bags if you do not want to wait for too long for the exact specifications. When my SA asked me for my preferences, I was very firm in terms of the bag size but I am open to the colour, leather and hardware. I gave a range of colours that I like and any of those would fit the bill. I ended up waiting for less than 2 months before my SA notified me that the Kelly I wanted is in stock! My take is as long as you're not compromising on your preference, it's okay (or maybe even better) to give a range of acceptable specs.
2. By multiple QBs, do you mean any of those would do or do you mean queuing for multiple bags at the same time? I'll take the liberty to assume your question means letting your SA know you don't mind getting a B or a K first and answer accordingly. If I were you, I'd let her know I'm okay with a B30 or K28 depending on which one comes first. In fact, my SA asked me if I would accept a K28 or B30 because they are more readily available (comparatively) than a K32 in my location. So I guess it won't hurt to let your SA know you like those bags and might even be contemplating getting them in the future. But of course, only if those bags are what you like. I would advise against compromising on your preference just so you could get a QB quicker. If your mind is set on having, say, a B30, I'd try not to talk yourself into getting a K28. The B and K are totally different designs and serve different purposes (imo) so even if the "2nd best" QB scratches the itch temporarily, deep down you'll still want your 1st choice.
But of course, like my SA said, you'll get the other bag (whatever that other bag is) eventually in the future.

Happy shopping!