Boise To Paris Connection

I'm about to buy the following economy flight online: AF8531 Leave Boise 1:10 p.m., arrive Salt Lake 2:17 AF9042 Leave Salt Lake 3:20, arrive Detroit 8:55 p.m. AF377 Leave Detroit 9:35, arrive Paris 11:20  I have a few questions: 1) If I buy from the Airfrance website rather than a third party, what do I gain? Will I get better help if I miss a flight? Better baggage allowance? Better seats? 2) Is the connection time of 35 minutes in Detroit sufficient? What happens if my flight into Detroit is late? 3) Im buying an economy ticket, the lowest price possible. Is it possible to change the return date? 4) Do you still sell "Open" tickets (with unspecified return date)? Or is that a thing of the past?   Thank you, RichardFaylor