Why doesn't mobile Safari have a JavaScript console?
You mention "standalone" so you may be aware of this already and have discounted it, but I thought I'd mention it anyway just in case.

It is possible to use the development tools within Safari on a Mac (including the JavaScript console) to diagnose faults and develop sites for iOS devices.


Connect your iPhone or iPad to your Mac using a USB cable.
On your iOS device, go into the Settings, select Safari then Advanced. You should see a switch for "Web Inspector". Switch it on.
On your Mac, go into the Preferences within Safari and on the Advanced tab you should see a tick box for "Show Develop menu in menu bar". Tick it.
Now if you open the Develop menu within Safari on your Mac you should now see a submenu for your iOS device listing the available "inspectable applications". I've only seen Safari on this list, but I assume other applications that use UIWebViews are inspectable too.


Arguably more useful than having web development tools built into the devices themselves.