Thanks for the detailed and prompt response - it's greatly appreciated, and no apologies are necessary. There are a couple of factors that are worthy of more discussion here, if you'll indulge me.
First is that, if the 'old' method really wasn't what was intended, you guys really need to make some kind of announcement clarifying the new position - not just posts in the discord or a one-line patch note, but a blog post in Steam etc. There are a lot of players who have been holding off playing for a month or so (since 1.2, basically) on the assumption that the 'new' method was a bug that would be fixed, and most of those won't see this thread or hunt down posts in discord.
Second is that while absolutely necessary, said announcement is likely to provoke a fair amount of backlash, and you guys will need to figure out how to manage that. I've got tons of respect for you guys and love your game, but even I find it a little hard to believe that a bug of this scale on one of the most talked about aspects of character builds somehow got missed for the six months since launch (and likely long before that - did no one at Owlcat with knowledge of the original design play a merged book this whole time?). Even if I give you the benefit of the doubt (and on reading your post, I do - albeit grudgingly), most won't, and mismanagement of the response when taking away something most players love could result in disaster.
Thirdly - I urge you to pass on to the developer team how much the players have become used to and enjoy this particular 'bug', and to encourage them to undertake that major rework that would allow it to be kept while the other bugged aspects are fixed. This late after launch, the 'old' method is almost canon and mechanically speaking, losing that power spike arguably defeats most of the point behind merged books - not to mention the way the new system works can result in players getting better/worse results depending on when they time their mythic levels, which is really unintuitive and likely to result in annoyed players.
If there really is no way to return to the old method, then I would encourage the devs to look for a middle ground that could mitigate some of the loss without requiring tons of dev time.
Finally, there may also be something odd going on with unmerged books, depending on whether they're tied to a specific stat (as implied by a list on the discord) or use the highest casting stat regardless (as implied by Fextralife wiki etc.). E.g. I have an Inquisitor (WIS caster) Aeon (CHA mythic book per the discord list) with 10 CHA. On his mythic book, I get a warning that I can't have higher level spells due to low CHA, but I can still cast the ones I have and the warning is still present if I respec the character to 16 CHA. Is the warning bugged? If the mythic books do rely on a specific stat, it would be helpful to state that in the info provided when selecting mythic power type.
Thanks again for taking the time to investigate and clarify the situation, even if it wasn't the answer most of us were hoping for (and apologies for the wall of text, lol).