Yep it was Kensei who turn full hollow, but if you remember when Love was fighting hollow Ichigo, Lisa asks about limits in training and Kensei told her Hiyori took 69 minutes and 2 seconds, the longest from all.
And other thing, Tosens fight against Komamura in fake karakura town, he released his sword into resurection form so he somehow changed his zanpakuto but it was the same sword which he used as shinigami.
Ichigos bankai couldnt be stolen because his bankai was inperfect, it was not in final form. Only bankai which cant achieve something else can be stolen. Ichigo just said that: "I wont ever ask for you to lend me your strength anymore, I wont tell you to get out of my way anymore, nor will I ask you to fight with me, from here on out I fight alone, thank you Zangetsu, for you are me". When you think about this words sword is me, hollow of other Vizards it is just them.
I Just think, inner hollow of other Vizards can awake in situation when they will dying. Ichigo is Vizard too at all, when they want control their hollow they must go on suicide mission and fight hollow in their inner world, they fill fight him and control him or they will become hollow. It is like Yhwach said when Oetsu was reforging his Zangetsu. In times when Ichigo was not able use his zanpakuto, in times when his life was in dangerous who saved him?
Other point: Do you remember what Kensei said Shuhei when he wants see his Bankai? We were fortunate that my bankai wasnt stolen this time. Person who not use bankai in fight will not say this words. And how Komamura, Hitsugaya, Soi Fong, Byakuya bankais were stolen? They used word bankai and bang.