Re: Visual clues for humidity
In general, yes. The larger, uniform-thickness guitar top may show more readily-observable signs of sinkage or "swellage" than the smaller, stiffer mandolin top does, but you're dealing with acoustic instruments made of the same or very similar materials. One of the early signs of guitar dryness is a lowering of the action, so much so that some guitarists have a "summer bridge saddle" and a "winter bridge saddle," of differing heights. Mandolin probably doesn't show this as quickly.
I would re-emphasize that having a hygrometer to consult, may allow you to adjust humidity before it significantly affects the instrument. Which is a good thing.
Gibsn: '54 F5 3pt F2 A-N Custm K1 m'cello
Natl Triolian Dobro mando
Victoria b-back Merrill alumnm b-back
Stradolin Vega banjolin
Sobell'dola Washburn b-back'dola
Eastmn: 615'dola 805 m'cello
Flatiron 3K OM