#1
I understand you have to intonate every time you change string gauge and all that, but what if you have a string gauge that lets you have different tunings , let say I go drop c first , and then I intonate. What if I go drop b with the same pair of strings, do I have to intonate again ?
#3
I normally don't bother. The scale length and the gauge of the strings haven't changed, so I don't see why the intonation would need to be changed.

One thing I do check is the neck relief though. That's something that actually does change with different tunings with the same gauge.
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I also have to do that. Cottaging this weekend