Yes we need a “back button” with this, you could double your current interval, and when you are done just hit next to get to the end of the interval. This would effectively give you an extension.
We had this request discussing this.
But maybe your ideas is better (simpler) which is just “extend interval X seconds.” We gave up on back button because it was too complex but extending would be much easier and you could put in settings your default extension time. 10s 15s 5m for example.
For VO2 work I am not fully understanding what the problem is. Why not just use resistance mode and shifting? Doesn’t that solve your problem? Again I think our automated slope mode which is basically the same as resistance mode except you don’t need to press buttons might help you in some of these cases.