I will say it's more Ian and Evan's fault for being bad at planning ahead. Evan specifically.
I wouldn't say any of them are at fault; it's just how things are with a brand. I wouldn't say either Ian, Evan, or anyone in charge of continuity wants to make continuity problems, but there are limits to what they can do, and there are things more important than that to higher-ups.
It's very clear from previous answers that IDW's transition into a Post-Frontiers world was supposed to be something bigger, but then you can have stuff like "We want X to be promoted at Y, so do your thing so you can add Y and Z elements to the comics so they can promote it" and at that point, all they can do is do what they are asked and try to juggle the problems of adding something that wasn't planned at the start. Stuff like this happens all the time with cross-media. If the higher-ups want to promote something, it's hard to say no. Personally, I totally believe that originally the intent was to close all the major plot beats by the 100 issue and only tackle Frontiers stuff after it (In fact, maybe they can even keep part of the plans in the end result and we can notice stuff like "If Sage hadn't been added, this would have been the perfect spot to transition into Frontiers").
And even from the point of view of the higher-ups, it might not sound like a problem. There are many cogs in a brand's machine and continuity is just one of them, they might assume that brand synergy and having the readers recognize a character (like Sage) from something they want to promote (Especially if this is supposed to come before the announcement of the follow-up to Frontiers), then preparing the seeds for that is more important than creating a continuity mishap, and if they think it's better to have the new readers come without the need to care too much about past continuity (In the sense of text done just to solve continuity problems), they might as well say "no need, that would create more problems to us than it's worth, just accept it happened and move on".
I disagree with SEGA's stance, but I can understand why it happened. I truly hope this can change, as I don't think even a single line would have made any readers decide to stop reading just because of that.