"Ripple"-delete for waypoints (ie, changing timing of following waypoints)?

[routeconverter]

Is there a way to tell RouteConverter to delete a bunch of points and to update the time-stamps on the points so that there's not a huge time-gap between the points which surrounded the deleted portion?

No

Or, if you can't do that, can you, at least, tell RouteConverter to adjust the time between two points and have that time-adjustment be made to all of the points after it, as well?

No

But RouteConverter is open source: you can hack the functionality into it