My VDO computers (over a decade old, do not recall model) have a way to put in a second wheel size for second bike. And they sold second bike kits with bracket and magnet. The theory was you could use one computer on two bikes.

If you know any really good experienced mechanics, ask them if there is any danger of your hacksaw job putting uneven pressure on the headset bearings if your cut was not perfectly perpendicular to the headset. And, is it a problem or not?

If you can put a thin feeler gauge in between stem and headset anywhere, that suggests a high spot that could be filed down.

Other than that, I think that was a brilliant idea. The bar bag is not very heavy, so the remaining part of your stem should be more than adequate.