Leaving aside the question of mental debilitation from the effects of his imprisonment, don't forget, he came from a completely different time, when the Nym were peaceful constructs performing agricultural duties. He had no more reason to expect an attack than you might had you woken up 3,000 years in the future to find a model airplane diving at you for a strafing run. Your first reaction would be to laugh and your last thoughts would be something like "WHAT THE FUCK?!?! They put poison dart-launchers on these things?" Someshta was something far removed from Forsaken experience. Yes he was a Nym, but he had three thousand years to master his power and build up and reinforce his connection with his place. They likely had no idea of his capabilities or any way of guessing. In the AoL, I highly doubt they were entrusted with maintaining a mystic sanctuary against corruption to guard special artifacts for long periods of time. The people of the current age look upon Someshta as something near lesser divinity - don't dismiss him just because the Forsaken know what he used to do for a living.

On what basis do you assert that? In the first place, there was no Callandor in that fight. In the second, it was a circle of at least three channelers, one of them a man of not-inconsiderable strength. Who says Eben's connection to saidin was fading? On what basis do you assert that the amount of saidin a person can draw decreases as they grow closer to death? For all we know, Daigian had access to every drop he could naturally draw (limited by the link, of course) until the moment his life flicked off. Secondly, previous calculations of strength are useless. I would guess you are going on Rand's assertion that he could handle any three Aes Sedai easily, and presuming the same applies to a Forsaken. Except Rand was assuming they would try linking to CAPTURE him, while Daigian would be using the strength at her disposal to blast away at Aran'gar and try to kill it. Not only that, Aran'gar would be more likely to run than Rand. Rand will stick with a fight because if he is in a battle, it is one the Light needs to win and the cause of the Light is what he fights for. Aran'gar is in this to gain immortality, and fighting to its death or a close scare is counter-indicated by its priorities. Once Daigian demonstrates sufficient power to threaten its life (and even without the augmented strength, she can presumably weave enough of the Power to kill a slender human being - this is not a video game, and the Forsaken do NOT have more hit points just because they are presumably on a higher level), why on earth would Aran'gar hang around and risk getting killed. It showed up like it was supposed to and can defend a retreat on those grounds as a tactic ("I was trying to draw the defenders after me so another Chosen could attack al'Thor" or something like that). The Forsaken are not in this to impress vapid readers, they are doing what they need to in order to get what they want, which is primarily "not dying." Sticking around to get killed is an odd way of proving competence at that goal.

Your humble opinion being worth what, exactly? Ba'althamel gets hit by an unprecedented and unrepeatable attacker shortly after he first wakes up, while Moghedian is repeatedly defeated by a single channeler, including on her home turf, where she is supposedly the greatest living adept, and ends up enslaved. Aran'gar meanwhile infiltrates and wins the trust of the very woman keeping Moghedian captive, and was prevented from killing her by the fact that she served the Shadow's cause better where she was. Moghedian lived on Egwene's sufferance at the very period where Egwene was living on Aran'gar's sufferance. Humble indeed.

Blah, blah, blah. Yes, if only it had not been smart enough to escape when its cover was blown by the arrival of a male channeler in the camp, which he could not have forseen, prepared for or prevented. In a situation where it should have been caught dead to rights, it still escaped. Compared to Moghedian being captured when she had Nynaeve and Birgitte helpless and unable to channel or shoot, or Egwene getting captured when she had the advantage of strength, surprise and superior knowledge of concealment and defensive weaves, your assessments are a joke.