It's bitcoin in that it's an identical distribution at the time of snapshot, so everyone that holds btc will be eligible to acquire an equal amount in btc-u. Is bitcoin not worth something because of the userbase behind it?
How can I mine BTC-U
You can't. There would be no more BTC-U created after the initial snapshot issuance, no miners to pay for maintenance as it will be running on top of the BTS DEX which is a self-sustaining DAC.
Does BTC-U has open source code on github and everyone can propose a change to it?
BTC-U doesn't need its own dedicated client (though you can create one) nor a dedicated blockchain, the BTS DEX (Bitshares) is entirely open source right now on GitHub and MIT licensed. Anyone can indeed propose a change to Bitshares via BSIPs and voting.
A dedicated client for BTC-U would be one honking good idea. Make adoption really really easy!
Wouldn't be difficult at all to do, you can easily customize and host your own BTC-U themed BTS DEX (web wallet).
Not only because of that.
Certainly not, however its distribution and established userbase is massively valuable, followed by its service/exchange integration then the brand awareness. In terms of tech specs, it's back in the stone age currently.