You actually can do some of what you want. 
To make Alexa reboot the hub, create a virtual "momentary" switch (use the built-in "Auto off" setting), then create a rule triggering from that switch turning on and send an HTTP POST to reboot your hub (like this), or use a community app to do the same. Then, to get the friendly invocation you want, create an Alexa routine triggered by that phrase that turns on this switch.
You can sort of do the "30 minutes" thing, too, but the big problem is that Alexa doesn't have a way to schedule such things by voice. This isn't really a Hubitat problem. If you always want 30 minutes, you could again create a virtual switch that runs a rule and does whatever you want, then create an Alexa routine to turn on that switch (but again, this would be limited to the specified time for that switch since Alexa cannot take in parameters like this; though if you really feel like it, you can use creative workarounds like manually/awkwardly manipulating a virtual dimmer and using that value for time).
Not so sure about the opening-a-Dashboard thing, but it's possible someone could write an Alexa skill to do that. I can't recall seeing anything like that for any platform. Maybe a good experiment for someone to try if they're bored. 
