[1] No. I don't think CODs innovations have much to do with its continued success to put it simply. I believe the game just needed to become as good as Halo, to effectively inherit its title.. and owing to numerous other factors (such as being available on all platforms, making it naturally more widely discussed amongst peers), it's had little trouble maintaining that status. Many of EA's sports properties for example are hardly the shining examples of constant innovation, but do you think something not branded FIFA is going to knock their game off its perch simply by being as good or even a little better? No... you'd have to be worlds better (like Pro Evo
used to be) to overcome its natural advantages. COD, Battlefield and even stuff like Medal Of Honor always had natural advantages in the market over Halo... but Halo previously was just so much better it overcompensated for them. That's as much in the hands of those games as it ever was in Halo's, much like Pro Evo's success would essentially be reliant on the failures of FIFA.
[2] We may as well drop this point tbh. You see the unification of the SP and MP as a fumble on Bungie's part. I see it as the reason Destiny is what it is. There's not really any way for us to move forward here, as a Halo equivalent doesn't exist. I agree that Spartan Ops is too phoned in to be a decent counter-argument.. but then, I'd probably argue the same with Destiny's entire initial campaign tbh, and that never stopped it snowballing at the time...
[3] I'm mostly approaching this from the angle of older CODs surviving past newer releases regardless. Other examples I can think of would have been Quake games through each release (as a contingency of each game's fanbase would typically despise the changes of each sequel), or Counter-Strike 1.6 when Source sucked. Halo's fanbase however moved onto entirely different IPs rather than continue to play the previous entries in any available form. This isn't really applicable to a story-driven single player FPS... but even then...
Half-Life 2 is really not the best example to bring up here. (check "Audience in 2 Weeks").
[4] Obviously is pretty subjective as to what's better or worse (I think every Halo prior to 5 is worse than it for example), but Halo 5 isn't competing against Halo 3. It's competing against Overwatch, Destiny, COD, Battelfield, etc. As per point [1], I feel that it's not "being worse" that's causing Halo to struggle to stand out.. it's everything else it competes against being a ton better than they were before. It's a lot easier to stand out as the big fish in a small pond before all the other fish get big too.
Retention would be based on a percentage. So If Halo Reach had started with say 250k and dropped to 100k, whilst Halo 5 had started with 80k and dropped to 40k, then 1 out of every 2 Halo 5 players stuck around, whilst less than that did for Reach. It's not a terribly useful metric (though apparently Halo 5's playerbase was also growing), but it's hardly something that's hilariously unbelievable. You're arguing with absolute values that aren't really relevant to on their own to player retention.. else Halo 5 could have started with 90k users, then had 95k users 8 months later, and you'd still argue that Reach had better player retention.
Also.. every single game in the genre is losing to Black Ops II on XBL right now...