Can't say about the safety as everyone over there is without mask. although dev deepawali is a surreal experience. if you are new I would suggest to stay around assi ghat aur dashashvamedh ghat for best experience and view of dev deepawali
I would suggest you to not attend this year's dev deepawali or even visit Varanasi until and unless the situation becomes a little stable as the real pleasure of exploring the city is when the lanes are crowded and as far as I know,people are being very careless these days without wearing mask.
Not it is not possible to stay at main humpi, you have to stay at hospet. humpi is just amazing place you can refer my blog for more information regarding humpi... :)