One APR showing has always been a good sign of approval. The showing a starting line with a preapproval is a fairly new development and I have never personally seen it. Considering that you did not receive a marginal approval based upon your $10,000 starting line on Cash Magnet, it would be unlikely for you to be declined after seeing a preapproval (and probably even with expanding/extending language showing) unless something had changed fairly drastically on your reports recently.
I have a short but intense history with Amex (first personal card opened last July, but I have four cards and have also gotten a lot of rejections over the last 10 months). For me, preapproval really means preapproval. I have never been denied when applying for a card for which I was preapproved. I HAVE been denied when applying with a "prequalified offer". I have been approved when applying without a "prequalified offer'. So for me at least, recognizing my profile is pretty weird, I do not take prequalification very seriously, but preapproval has been 100% for me.
Another nice thing is that if you apply and get denied, it will be SP-only. Even if approved, it may well be SP-only. Those are not guarantees, but are very likely, and that changes the equation quite a bit on when an application is worth trying. I also don't think applying and being denied hurts your internal Amex standing, at least, it has not seemed to for me. Example: I applied for Platinum on Jan. 10 without preapproval, denied, got a preapproval on Jan. 17, applied on Jan. 18, approved.
I think you would be approved for the card, but the CL and APR will be unknown until you apply. JMO based on my personal experience. Good luck!
