If it's possible I think per site settings for data savings mode would be very useful, for example I could set Facebook, Amazon and some other sites to always open in high data saving mode while all other websites open in extreme mode.
That may be the case, but the browser used to have a much smaller footprint - ideal for lower spec devices. I've used it for a long time on various phones, and it has never used so much storage as it does now.
I don't want to disallow Mini from accessing my intranet. My concern is that Mini will not render anything as it will definitely not be able to reach the Opera Transcoding servers.
Russian ISP may have a proxy set on your internet data account, check that and keep an empty field there. Though, Yandex campaign could detected the DNS only beside IP range so it's better to change the DNS too.
Hmm, after further research, I found that Opera Mini (v15 & v16 beta) only works with 64bit versions of this OSes (tested with Remix OS, as Phoenix OS doesn't have a 64bit version yet).
Well, at least that's a start...
Let's see how it goes...