Sure, a team of pro cyclists, even without aero gear, could average 30mph. I have personally been on a flat circuit course on a regular road bike where we averaged 28mph for an hour, and I'm an awful cyclist (pro cyclists can put out about 50% more power than me). But the chase group could easily ride faster than that if sufficiently motivated. The leadout reaches 40mph towards the end of a flat stage. The only reason the fastest schedule is 43 km/h (excuse the change in units) is because they're not expecting them to go full gas the whole time. At 43 km/h I can ride in a pack forever and chat with other cyclists, I can even take short pulls at the front at that speed.
Following your numbers:
1. If a threatening group attacked from the gun, the peloton might be surprised, but let's say the peloton is riding along at 40kph and the breakaway is going 50kph.
2. Say it even goes like that for fifteen minutes, which it won't because they could immediately pick up the pace if they wanted to, but let's say it does for some reason. The peloton could just start riding at 55kph for 30 minutes, or even 60kph for 15 minutes, and shut it down, and once the break is caught they're never going to get away again. The breakaway can't ride 60kph indefinitely but the pack basically can.
3. The sprinters wouldn't mind the pace because they're all suffering together, and winning the sprint is more important.
Like I said, breakaways only escape on the flat stages if they're huge or if the pack decides not to chase.