I checked the balance on the private key (the one with the smallest amount), 0 BTC, address never used. Then I checked it based on it's legacy public address, again 0 BTC.
Chipmixer has abandoned legacy addresses since July 2021, it only gives you the private keys for the SegWit native addresses.
Know of any javascript sites that work offline to calculate the segwit address from the private key?
Is there a specific reason you want it to be a javascript html page? Why don't you just import them into Electrum? If you really want them to be legacy, you can just import the private keys as p2wpkh, generate a legacy wallet and send them there, which you can then check from brainwalletx.