(1) what data Opera had about the hijacking of its browser engines that led to the limitation in default engines offered, including how exactly the hijackings took place. In that regard, was there a uniform method, or might there be a number of different ways the hijackings occurred?
It's not important what data. Result was locking(signed by opera, and checked in runtime) of file
search.ini in Opera Presto, and
default_partner_content.json in Blink Opera against changes. Not files with users private data, but with default search engines. Suggesting, that even changing of password data are secure, when they decided to lock only search engines. Also protection of file system is not function of browser. And even if it's nice function, it can't be used as argument to restrict things. Then it's more fear factor, than argument, because then we're dealing with software able to access file system. And then there are much more important things to protect, than default search engine.
Also default search engines are in Chromium stored together with other custom engines in file
Web Data. Opera did own mechanism. Yes if was maybe in time, when even Chromium hasn't default search engines there. Even so they could simply encrypt default search engines data in this file same way like they do with passwords in file
Login Data. They made it more expensively. So mechanism used in passwords was not safe or was it more important for Opera to secure search engines file, than user's passwords data? If it's not, why did they did it harder and more expensive way? Or they care more about search engines security, or they don't trust mechanism used while saving user's password data. It they care more about search engines, then this security argument can flush itself. It they don't, but they don't trust user's password data saving mechanism, they also gave higher priority to deal with search engines against user's private data.
Still there was easier way how to deal with this problem, without much costs. Or the real problem is much bigger, but in other place.
Even reasons are money based. Or they made really really bad decisions, that don't make much sense. Or maybe they started with good reason, to "correct and enable later", but in time, this is still just better "solution". Still they didn't care to change anything about it in year and half.
(2) how default engines (other than the ones approved by Opera) contributed to it, that is, made the hijackings easier.
It's not about how many of them is, but about mechanism, how they are stored. So mechanism can make hijacking easier, not count of engines.
(3) does Opera view its default engines as inherently safer than others would be if they made them available, and if so, why?
As we know this engines except Wikipedia are paying to be in the list. And as I mentioned before, some are not even set correctly. And there are safer engines. So there's no "good" answer for Opera.
(4) or is that Opera needed to offer some default search engines to compete with other browsers ( maybe even made income out of the default search engines they offered), and that while all engines can equally (and as easily) be hijacked, that the risks of hijacking become greater if you offer more (hence a limitation on the number Opera chose to offer).
Sure we know they're getting income from searches. Sure every browser now offers default engine due to money reasons. Yet it's not reason to restrict change of it. Risk is about mechanism. It has nothing to do with number of engines.
(5) or is it that for some reason it would help if we knew, typing the code would make any search safer, but Opera did need to offer some default search engines (to compete with other browsers) but wants to limit the risk of hijackings, by limiting the number of default engines offered.
Typing of word safer? Really? Then if this is reason, they should disabled default engines at all. It's not making sense. They are not limiting risk by numbers. But by locking mechanism to change engines.
(6) why typing of the one or two letter code, is in Opera's view safer.
Who stated that this is safer? It's nonsense from technical point of view. If this would be reason, then they should change omnibox behavior, add search box, .... . Not has opened API architecture.
(7) why via extension Opera lets Disconnect Search get around the limitation for DuckDuckGo, and any others in the Disconnect Search engine not in the Opera approved default list. (Surely Opera concluded this extension provides needed functionality). Maybe there's a liability concern. If Opera provides the additional engine, they are more responsible for safety than if a third-party provides it, where their review is on its face lesser.
Because it's by design opened API. Just like some extensions can change things, this can override search engine.
And responsibility? So in the first place it's responsibility of user, not extension. This is question of some "Terms of Service", not any restriction of this kind. It's alibism.
And if you care about users security so much, you allow extension to compromise it, and you don't care, because you're safe by law while on the other side using security as argument? So (if this would be true) then yeah, this is approach users should love.