As expected, got booted off Adamantium League - but not for the reason I predicted. The minimum score for Adamantium keeps going up, so it seems I have to grind a bit more to get into and stay in Adamantium League.
The upside to all this is that the combination of my level (now 189) and my ELO score of 1684 (yep, just got back into Adamantium League ^_^ ) has translated into AFK challenges reduced to 1-2 a day. What this means, in practical terms, is that on a bad day, I will lose 50-60 ELO, and on a good day I lose nothing at all.
I can't tell for sure if this is intentional, or if it's related to the match pairing system in place. I suspect it's the latter. Either way, it seems that higher-level players are "rewarded" with having to face fewer challenges once they get into a particular Ranking range - and needing less effort to stay in Adamantium League in the days to come. The problem, then, would be getting there in the first place, and then getting high enough that AFK losses won't be such an issue.
In the meantime, I continue to use my Blade Rush build. I generally only lose if the opposing agent goes first - common for my level range, so I'm not too worried about setting myself back several fights. As long as I can continue to grind, and as long as I am able to make up for any losses, I should be okay.