Thanks!

It is a bit tricky that SkyDemon has slightly different menus etc. on different platforms.

There is no equivalent function on the PC version of SkyDemon. However that function isn't really for setting an alternate; to do that, right click on the relevant airfield and choose Set as Alternate from the menu that appears (or press and hold on Android/iOS).
Thanks for letting us know about the issue with other sectors being replaced when you use that function, however. That sounds like a bug and we will investigate.

In the Android version of SkyDemon, there is an "Edit Route" menu item that opens a box where you can textually change details of the route.

Question: How do I access this function on the PC version of SkyDemon? As far as I know this function is the only way you can add an alternate airport after a route has been created. I am using SD 3.2.1.0 on the PC.

Problem: If I use this function on the Android version of SkyDemon with a multi-flight route, all flights except the currently selected one disappears. (Even if I only change the route part and not the departure/destination airports.) I am using SD 3.2.1.22629 on the Android.