All railroads are on grades. There's really very little that is 'level' anywhere. Even bridges and turnouts are on grades.
The difference between them and our scale models is the real distance we have to work with. A prototype turnout might be a whopping 60 feet long when the crane lifts it off the flatbed and into place. Even on vertical curves at the base of a grade, the distances involved are enormous, something like eight or ten miles in HO scaled trackage. We have to fashion the same thing inside of 12-18 inches. That's where we run into trouble with wheels lifting or pilots/snowplows snagging and rubbing on vertical curves that are too sharp.
You can let your flex track sag naturally and ballast under it to fill and to support it. If you have to insert a turnout there, you'll have to pause the grade or gently force the turnout into a couple of degrees of accommodation. That can cause problems, but if you keep it minor, you should be okay. The turnouts are NOT designed to sag, but you can impart a minor 1/16" sag near the frog if you are careful. I'd advise against it. It won't be the through route that will cause the problem; it will be the diversion at the depressed frog that will cause you derailments at least 40% of the time, if not double that, and it gets worse the longer the frames of the leading head end power.
And yes, the longer your vertical curves to get into a progressively steeper grade, and they are necessarily longer, the shorter the grade between the two vertical curves. You can't have your shallow grade cake and still get the rise you need in a short distance. Or, you can't have your steep grade cake and not have longer curves at either end...meaning you still have less rise when you're done.