Let me investigate about this parameters and as soon as I can get some information I will post it here.
Usually a tolerance of +5 % should be safe. Ii guess the 1.8V inputs will also stood up with 1.9V.
But never with 3.3 or higher voltages ! For this you can use f.e. a level shifter which converts between 1.8 <-> 3.3V or 5V.
I think it could be much more than +5%.. ( Or less....Who knows).
"less" doesn't damage anything - the most important
You are right, normally +5% is a conservative givenness and there are a lot of systems getting along with let's say 10% tolerance and maybe more. Pleasingly but never a reference, in relation to microprocessors it's better to be very careful.
It should follow the supply voltage VDD1. Ie max =VDD1 min = -0.3V.