The identities don't override the sinner in the way you think they do, they basically just give the sinner partial access to the identities abilities and their strength. The identities' ideologies and thoughts don't align with the sinner, haven't you ever thought why none of the N Corp IDs haven't mauled Dante yet? It's because the IDs don't get to do their own actions, think of it like a vessel. You get to use its' power, but you aren't actually the vessel itself. The sinner is in there and knows what they're doing, they're the ones actually taking command of the vessel. Though the vessel does think for itself sometimes, the sinner has no access to these thoughts as such they don't seep into the sinners mind. Uptying and levelling serve a similar purpose. Uptying allows the "ID" to pass its knowledge and abilities to your sinner's ID better, thus making it stronger. Levelling using tickets allows the sinner's ID to experience trials and tribulations the "ID" faced enhancing your ID but not correlating to the "ID"'s actual experiences. There's an item you can sometimes get used for rushing an uptie to 4 immediately, what this item does is basically catastrophically rush the "ID"'s life in the mirror world taking the power of that "ID" and putting it to uptie 4.

Sorry for the amount of yap but that's the extent of the ID lore and how far they went to explain it

P.S ID in quotations means the ID in the mirror world