It's the AFHPSP, so I'll answer the one's that apply to all branches. I'm USN but having explored all three and since no one else has yet to respond...
1) Get it in writing (this is lower than USN and USAF "Field Select" Quals), and it is a conditional acceptance awaiting (A) acceptance to medical school, and (B) physical qualification--the former is the hard part as the latter is ripe with waivers for future physicians. Furthermore, "field select" status is not that big of a deal for USA and USN in particular. With the state of applications from recent years as a guide, the odds are heavily in your favor that, if you get into a medical school and wan't to do HPSP, you will get a scholarship. USA and USN have not filled their quota in recent years.
2) In a sense, yes, but look into it. If you don't match you will have to do a GMO. More prevalent in USN and USAF, but something to explore with the Army if you're looking at a competitive (residency) field down the road.
3) Depends. All branches incur active duty obligation (ADO) for both time on scholarship (year-for-year but a minimum 3 year commitment; thus, only year-for-year in certain cases, albeit the majority) and for time in military or full-time inservice residency. These ADO's are served concurrently such that the better way to think of it, and the way it is phrased in the AFHPSP Contract, is that your active duty obligation will be time on scholarship or time in residency with the military, whichever is longer. For example, a 4 year HPSP student who completes a General Surgery residency (5 years) will have a 5 year active duty commitment (not 4). Additionally, if that same student were to do Family Medicine (3 years) they's have a commitment of 4 years. Additionally, you will have 4 years on Individual Ready Reserve and these commitments are always subject to "stop-lossing". If you can, see if they'll give you a copy of the AFHPSP contract to look over at your own pace, not in the 40 minutes before you're commissioned.
4) Yes, you supercede to O-3 with 4 years Entry Grade Credit, more if you have an additonal MS/MA/PhD in certain fields. The latter affects time frame for promotion, either way you're a captain when you come out as an intern.
5) $1907 /month before taxes this summer. This is paid while you are in school, not during breaks--really only an issue after M1, but you'll do Active duty during that summer that will pay at the rate of a 2nd Lieutenant.
6) This is absurd, perhaps they meant to say residency (as that is the common pitch)? You are not on the Army HPSP until accepted to medical school in the first place. The only area where it may be remotely true is if you are waitlisted. Here your ability to pay may be a factor. Accordingly, having an HPSP (and obviously notifying them as such) may help in that you would not require institutional funds. This is a non-issue at most schools, but holds a drop or two of water at some in the case I've described. Others, I'm sure, will point out the equally illogical nature of this boast.