carry on luggage requirements

Why don't you get a real carry-on with no wheels. It would weigh half of what you are considering. Eagle Creek Adventure Weekender might be a good choice, and there are others. I would consider any bag over three pounds too heavy.

I certainly agree with Monte, ditch the heavy wheels and rigid frame. I have been using the Essential Carryon. It weighs about 1¾ pounds and, as long as you don't stuff if full, will "deform" to fit the sizing box used by many airlines. Since you are limiting yourself to the the 1960 CI for the rolling backpack, it shouldn't be difficult to keep the content weight down, and the bag adds less than 2 pounds to that weight. Remember, every time you have to lift and carry the bag - up stair in hotels or train station, into the overhead bin on a plane or train - if you have a rolling bag, you'll be lifting almost four pounds extra. Depending on how you plan to get to Europe, you might be limited to less than 18 pound total weight. Starting with a 5½ pound bag doesn't help you to stay within the limit.

Wheels are great in Europe. I would suggest you look at business luggage. I have a Delsey that would fit in the requirements you listed. It has often been my only piece of luggage...except for my purse. Although it has a shoulder strap rather than backpack straps, it is lighter and its rarely necessary to carry suitcases anyway. But if needed it can be swung crossbody or just over the shoulder, so hands remain free. The rest of the time you can roll it, no schlepping for me. Even when I used to use backpacks I always packed wheels.

I am just the opposite. I find wheels to be a real pain in Europe.

As others noted, airlines commonly have frames or boxes of sorts that are a go/no-go check of the size of your carry-on. I have a RS convertible w/o wheels and have filled it to max-plus. The advantage of not having a rigid frame or components is that I can stuff the bag onto the frame. If you must have wheels, consider that the wheels and associated hardware add weight. Europe and other areas outside of North America may not be mobility impaired friendly. Wheels don't work well going down or up stairs nor roll easy on cobble stone walkways.

I've noticed it's often men who are most adamant about wheels being a bad idea. Some of us actually appreciate the assistance of wheels and the freedom of movement that they bring (as, I would guess, will most men at some point in their lives). And I've had absolutely no trouble with wheeled luggage on cobblestone, brick, etc, in more than a dozen trips in Europe and the Middle East. On hotel stairs, I just pick it up and carry it. On stepped bridges (eg, Venice), I usually just wheel it up and down the steps. One thing I've done when I was worried about having to check my bag (even though I was complying with requirements, I was worried about a surprise, and I couldn't risk losing my bag)... I bring a tiny, very lightweight duffel (Sea to Summit brand, less than 3 oz, but zippered and huge). And I pack my luggage so that I know exactly what I'd up in the duffel, if forced to check my main bag. It really helped when I was required to gate-check a bag that met carry-on requirements. I put almost everything in the duffel (which could be forced into less space than my bag), and checked a bag that only had things like books, optional papers, flashlight, sandals and sunscreen I didn't need in the weather, etc. So when that bag got misdirected, nothing essential was lost and I could get along without it for a while. It eventually caught up with me.