The back is almost certainly plain plywood and definitely structural. You can’t just cut it away and slap some doors on and call it good. A good cabinetmaker may be able to do it but they’d basically be rebuilding it and the finish on the back side would be difficult to match to the rest. I’d be surprised if the cost didn’t match or exceed ordering new. I’m sure you could reuse the doors though.

(I’m not a pro but DH and I are experienced DIYers that have built some simple furniture pieces and installed our kitchen cabinets)

I am so curious what exactly you mean? Do you mean insert glass on the ends that are currently wood?

you could place metallic wallpaper on the backs/ metallic paint on the back/ install cut to fit pieces of mirror on the back. A peninsula upper cabinet will have front and back glass doors...and each of them opens to reach in. You would find another way, with this cabinet, to have a glass or mirrored appearance applied on, as the back is installed to a wall.

I have no idea what you're talking about.

Sure! Here you go.

do you have a plan w a contractor to take out your wall, construct a peninsula and repurpose your cabinets?

It could be done but likely at a higher cost than just purchasing new cabinets.

Biggest obstacle would be matching the doors. (Walzcraft has some designs that are very close, close enough.) Inset doors require more skill to hang.

Other cost factor is painting because it will be impossible, or next to, to match the existing finish. Judging from your inspiration pix, looks like you're planning to paint them anyway.

When you say they were purchased by you from the previous owner, do you mean they were set up like that in a house you purchased or you just bought cabinets from someone and you’re removing from their house to put in yours?

You are trying to create peninsula cabinets out of the existing. Not a DIY project. You have to take them to your local cabinet maker so they can either copy the doors or create cabinets from your existing. Doable but expensive.