I realise that you're using a one-piece cover, but the process you're using is still part of a 'wizard'. When you upload your one-piece pdf, it will be previewed in a window showing the green border - as you've seen. That's part of the cover stage wizard. The fact remains that the edges of your cover should be behind the green border because that's (roughly) the area that will be cut from the final printed cover. So for instance on the book I made recently, the height of the cover I had to create was 9.25 inches - as identified by the wizard - though the actual height of the book, and its cover, is 9 inches. Obviously .25 of an inch is likely to be cropped, .125 from the top and .125 from the bottom. Roughly.
As for the centering of your spine, that's more difficult, especially as you've chosen vertical text. If you look at most books you'll see the writing is actually at right angles, unless it happens to be a particularly thick book with room for vertical writing. In effect all you can do is put your text in a text box and manoeuvre it until it seems to be centred. There's a possibility of about an 1/8 inch offset for the spine anyway, which Lulu says is typical for most books, not just theirs. When you've finished the publication process and uploaded your cover, you'll see at the last stage an indication of the front and back covers and the spine - that gives an approximate picture of how it will look. You can play around with the image if it still doesn't look right. However, the only real proof of the pudding is in the printing.