1 hour 40 is plenty of time at FRA. You’re right, it could get delayed and miss a connection. Your original could have been delayed long enough to miss the connections also. There’s no point I ask in what if, especially when there’s not much reason to worry (ie, impending weather that is likely to develop into IRROPS). Life happens and your wife will get to NCE. Hopefully on the original connection. But there are other flights later if she misses it. Sucks if it comes to that, but everyone on here has dealt with it at some point.

I disagree slightly - you can ‘demand’ anything, but it doesn’t mean one will get their demand, especially when any re-route is at UAs discretion.

I agree that demanding is rarely (if ever) the right tactic, especially with an airline - I find being nice and phrasing these as requests, using words like please and thank you, smiling, etc. often gets me the outcome I prefer. Not always, but often. If I were the OP, that’s what I’d try.

but the semantics does matter. If you go up to an agent and say in an angry voice ‘put me on LH flight XXX now’, you’re likely to have the agent respond in kind. If they’re really nice, they’ll just say no I can’t, you’ve been rebooked with plenty of layover time, without even looking. At worst, they’ll respond in the same tone. Do you like to have soemone use a demanding tone right off the bat? If you go up and nicely ask something like ‘my flight has been rebooked, but I’d prefer to have a longer layover in FRA, and I see LH has a flight leaving at about the same time as my original, can you see if there is a way to put me on that flight, please’ they are much more likely to give it a shot and see if they can rebook you.

replacement flights happen all the time, I wouldn’t worry unless there is antrue reason to believe the replacement flight coild be delayed (showing late inbound, expected weather, etc.). Again, if you want something else and ask nicely, and you’re more likely to have the request granted.