My question is, was it REALLY forcibly required to give consent to use the new forum engine
The request for consent is legit. It wasn't there with these new forums at first and only happened somewhat recently with changes in European laws. Nothing to worry about.
Opera Mini is different and for low-end devices. But Opera Mobile was preloaded on my Galaxy phone and it seems that now it's converted Mini silently without any prompt by an update. I just wanted to confirm this.
Shouldn't i first uninstall Opera 12, then install Opera 25 which is generally advised for ages?
There is no need to uninstall Opera 12, it can live together with Opera 25+ without problems. As said, just install it in a different folder if you don't want their files to be together in the same folder.