Nah. Strahd has noble qualities. But he's still an evil vampire. Of course when it comes to picking alignment you have bear in mind that it can change over time. Loghain was LN earlier. And if you redeem him he reverts to LN. But when he was hiring assassins, selling people into slavery, conspiring to kill prominent nobles, abandoning his king and y'know, just not killing Howe, he'd fallen off the path.
Well, again: your argument for abandoning Cailan being evil seems to be rooted in the assumption that Loghain either couldn't see any of what was going on and knew nothing except for when the signal goes off (in which case I suppose I agree he should have charged when it went off,) or could see everything he needed to in order to charge. (In which case I think we can agree there was no purpose to the beacon and Loghain should have charged when necessary.) As I'd previously noted, that doesn't sound like a safe assumption to me. There's a whole lot of tree cover, and if he wasn't some distance away from the fight the darkspawn probably would have noticed him, so fine detail seems like it would logically be lost. On the other hand the darkspawn horde was either conjuring giant flaming balls of light to march with them or just carrying torches, so that there is a super-long darkspawn column with (so far as we the players have any reason to believe) no breaks in it is something he could reasonably have gathered. The sort of forest that we see around Ostagar doesn't seem like it would prevent him from seeing that there is such a column: as I recall it seems to be trees spaced far enough apart to allow men to march through them a-dozen-abreast. He might well be unable to pick out a good time to charge without being unable to recognize a bad one, which would absolutely justify the beacon being used. And if Loghain really does need the signal to pick a right time to charge, and the signal is a large inextinguishable bonfire being lit, he knows that he can't pick a right time to charge now.
To build off of that, one major benefit of such a signal is that whoever gives it (Alistair, whoever he's watching for, or both) can watch for darkspawn stragglers that can flank Loghain's flankers. If Loghain is using the beacon for that, and when it goes off Loghain can see that there are still darkspawn pouring into the valley, then Loghain knows that he's not going to get a properly timed signal and has reason to fear he will get flanked if he charges into a seeming-opening without one.
Note that this isn't me denying the rest of your rationale for Loghain going Evil. The stuff after Ostagar gets progressively harder and harder to justify. (Well, depending on what order you do what in, anyway: I usually do the Circle first, in which Loghain's main failing was the error in judgement in which he trusted Uldred not to bork things for him. Though I think we agree there since you didn't list the Circle.)