They are temporary - for both probabilities …
If the calibration is fundamentally sound, but “something has happened during the flight” (might have been an overload in extreme conditions), there will be no requirement for an adjustment after reboot because the calibration is OK.
If a calibration is actually needed, the “tilt” will return after a reboot … because a recal is still required.
This post above gives a bit more logic behind this.
The only calibration I tend to do if I’m flying a new location or I haven’t flown in a while is a compass calibration as this will make the return to home more accurate 
Same. Unless the gimble is way off.
It helps more now I have the peli case as I don’t have to put the inspire into transit mode and take off the camera.
Yet to do a compass calibration with the MP in almost 4 years. It’s never suggested one is needed.
Not really, the home point is either recorded or it isn’t, hence the ‘please check it on the map’.
I’ve calibrated the Mini more times in the 5 months or so I’ve had it than I ever did in over 1000km on my other DJI stuff (because it keeps making me)
I did the compass calibration on initial set up out the box but not had to do it since but that may be because the places I’ve flown so far have been no more than a few miles away from home. I think I saw somewhere that the compass calibration may be required if you’re a longer distance from where you last calibrated it? Not sure how true this is though 
From the UK to Germany, Switzerland, ALL of Italy including Sicily … never been needed. The magnetic field right across Eurpoe (and further) varies by less than the sensitivity of the compass(es).
Ah ok so I guess wherever I saw that info it was inaccurate then 
I’ll wait to see if or when it asks for a calibration. I have to say so far the RTH has been highly accurate, I’ve been really impressed by it
RTH is as accurate as the HomePoint.
If the compass is off, all that changes is how direct the route to the home point is. It may circle in, slightly, as it gets close - until it’s at a bearing from the home point where the error makes no difference, and then it will still get to the home point.