1) The lifters are a carryover part. I don't know if any changes have been made recently.
2) As far as I know, it was purely for NVH. I haven't been involved too much in the gears this time. I know we looked into them for the 2019 but gave up; I think the new design has a higher number of teeth.
3) I honestly don't know. If I were guessing, it was cost and complexity...CGI's a PITA to machine.
4) I'd wager it's to warm it more quickly as the video mentions...and that's only for the pickup. The cab chassis version has the 'normal' outlet elbow with the DOC mounted further back.
5) Being at the plant, we don't have much exposure to the aftertreatment system. The only change I know of is a little SCR doser coolant pump that's mounted to the cab chassis engine.
6) I've not seen any taken apart like I have a CP4. I know the low-pressure pump on the rear is gone (the little gears would eat into the sides of their case on the CP4 IIRC), and I know something'd been changed with the pistons. Supposedly the CP8 performs as well or better than the CP3 in durability testing.
Unfortunately not. That's the first thing we asked.
Engine braking is just like it was on the previous engines, it's done through the Variable-Geometry Turbo. It's a standard feature. Gassers don't need that because engine braking is a 'feature' of them...just downshift a gear or two, like you mention.
The starter's a standard starter, mounted to the transmission adapter. It's the same part as the 19-24. As far as I know, it's just a 'normal' alternator (I know the 2-pin connector's the same). There are a couple of dual-alternator options as well.
Sorry, unfortunately my knowledge is somewhat limited in scope. But I try.