By the way: 3 minutes with google gave me a promising result for rooting the phone (untested and without any warrenty of course http://4pda.ru/forum/lofiversion/index.php?t565896.html)

Some Chinese vendors integrate malware intentionally, not by accident. Of course, they do not assist you in removing it. However, this is not related to opera, even if it is called opera service.
I think the right place for this question is an android or a malware forum, but definitely not the support forum of opera for android...

This is no effective protection against trackers. By simply using tor, you only hide the relationship between IP addresses and Cookie/tracker-IDs. So the only thing that is improved in regard of tracking is your current IP address.
Butthanks for the idea, I did not know Orbot yet (but Tor)

As far as I know, there is no account linked to an opera for android installation.
That means, that the saved pages are not stored online.

So if you formatted your device without prior backup, most probably those pages are lost.

Opera mobile has an urlfilter, which can be configured using opera:config

opera:config does not exists in opera for android, and add-ons like
https://addons.opera.com/de/extensions/details/opera-adblock/?display=de
cannot be used as well. Is there any way to block trackers and/or ads in opera for android?