The BF says they haven't been told when the reservations will start being bookable. Only that it's the only way to get one in store for the foreseeable future.
And yes the length of time in each slot is so that you can try it on if you haven't, pay and pair. And yes they should have additional bands to buy if you wish.
The only catch is that you can only reserve one model and if you decide you hate it you have to start over with reservations. They won't have set aside atock for you to just change your mind. Given that folks can book from home it's unlikely you would get lucky and what you decide you want is there unclaimed. Or you order for home delivery.
I live a few miles away from my nearest Apple Store here in the UK in Birmingham which is only a small store anyway. Because I had already planned a trip with my friend to the shopping centre on the 24th I thought it would be a good chance to pre-order online on the 10th and pick up in store on the 24th, I know which Apple Watch I want, the SS with a blue leather loop and 2 sport bands (white and black), but now I'm not to sure on the chance of getting one for the 24th if I do this. The situation I had with getting my iPhone 6 Plus was a pain, I went to the Apple Store and they had sold out so by the time I got home to order online shopping times had slipped so I ended up waiting a few weeks.
If that's going to be the case with the Apple Watch I'm thinking of pre-ordering and buying online, but it would be nice to try on in store as its a diffeerent kind of device to the iPhone and iPad.