How safe is to use 700x25-32C tube on a 32C tire? I know this question begs a clear "of course" answer. But is there any drawbacks stretching the tube to its maximal proportions provided by manufacturer?

Are tubes even more elastic/stretchy so using maximal width isn't nearly as much as this tube can handle? I always worry that inflating the tube to 80-85psi and maximal width can lead to more frequent, irreparable flats.
It is completely safe and shouldn't be of concern.  The only existing questions I can find discuss using an undersized tube (18-23 in a 28 tire, 23-25 in a 32 tire) and even those cases might work.  (If you've ever pumped up a tube to find a puncture, you've seen that tubes typically can easily expand far more than their rated size.)

The tube doesn't contain the pressure; the tire does that, so max pressure is purely a function of what the tire is rated for.