Again, we have no solid proof that Grimjow was in fact still an adjuchas when he was turned into an arrancar. I tend to believe that he was if not a true VL then pretty damned close when he was turned into an arrancar soley BECAUSE of his released form. I would venture to say it is a good bet that from Grimmjow on up in the espada were VL class hollows/arrancars also because of their released states and the power they wielded. I thought I remembered an interview or something done with Kubo where he said something to the effect of that there were definitely VL class hollows among the espada. I could be misremembering but I believe that was the case.
Grimjow was an Adjuchas class hollow, yet his release was very VL-like. After transformation he retained many of human characteristics, even though he was a complete animal as a Hollow. It's quite possible that this Hollow looked more like the form Ichigo took when training with Vizards, but after the Hollowification process, he gained partial human traits.
Yeah that time where Ichigo's was in full hollow mode, I don't think you could really say that Ichigo's hollow actually took over. As you said it would have behaved more like in the fight with Byakuya. It seemed more like his own power took over and was just running on Ichigo's instinct which was to kill the MF'er standing in front of him and to protect Orihime.
It's worth mentioning that all Adjuchas and above have regained a sense of self, while the moment that Ichigo became a full hollow... well he was far from being sane, neither did he seem to display a higher level of intelligence, thought it was able to mutter a word or two, but other than that, it was no different than a beast, which only the Gilliam subclass of Menos Grande and a few of the normal class tend to behave like. Of course it's quite likely that it was like that, because in this case Hichigo was merely a representation of some portion of Ichigo's being and wasn't fully him (it's possible that if he was to truly take over his host, it would behave more like it did at the fight against Byakuya).
Yeah we still don't know enough about this particular hollow to take a good stab at what it's motiviations are, but those guesses are as good as anything else. It would explain some of it's peculiarities at least.
It's worth noting that this Hollow doesn't behave like normal Hollows, it doesn't target those with high spiritual pressure, but specifically Shinigami. This might mean a few things, like:
- it has a grudge against Shinigami, which would make it a normal hollow of a former Quincy, seeing as Adjuchas and above level hollows have no longer memories of their past (they are agglomeration of 1000 of 1000 souls after all), unless it attacks Shinigami because of what Aizen, a Shinigami, did;
- it is under command of Aizen, but that doesn't really seem the case, it's seems that it was merely pointed to that location, rather than given specific orders, beside something that took down Ishin, would be able to take down Aizen as well (unless he used his magical sword);
- it targets Shinigami for the sole pleasure of hunting.
We need to remember that VL are highly evolved beings by themselves, they do not need to eat, drink, fight, they tend to seclude themselves in the deeps of HM. It would be strange for VL to randomly attack Shinigami in the ER, unless it is forced, like Espeda, or has other reasons.