I assume you are hypothesizing that the Rockies move further east, effectively splitting the North American continent into two. I am also assuming that this process is happening though the meeting of two tectonic plates as it is now. If this is the case, the answers to your question are:
For the first question, yes the Midwest would still be prairie but there would be a key difference, namely it would be the short grass prairie of Montana and Western Dakotas and not the tallgrass prairie of Illinois and Iowa. Basically, mountains create what's known as a rain shadow. As moist air moves towards a mountain, it rises becoming cool. Condensation happens, rain happens. Rising higher and higher, more moisture condenses and falls as rain until there is barely any left in the air. As the rain passes the mountain, the leeward side gets barely any rain and is now quite arid and desert like. One can see this in the Pacific Northwest with the Cascade Mountians; Eastern Washington, Oregon, and BC and Idaho are significantly drier than western Washington, Oregon, and BC. So basically, as the rockies move east, so does its rain shadow and the great plain effectively begin to look like eastern Montana and the Dakotas: arid, high plateaus.
For the second question, I am less certain of the science of tornadoes so take my ideas lightly. Basically, two things could possibly happen. One the one hand, moving the Rockies east could destroy tornado alley. This is because tornadoes need warm moist air which comes from the Gulf, cool dry air which comes from Canada, and warm dry air which comes from SW USA and Mexico (link).
Moving the Rockies east could cut off the supply of warm dry air from SW USA and Mexico, killing off Tornado Alley. On the other hand, if the Appalachians still remain and moving the Rockies east does not cut off the supply of warm dry air, it could potentially make things worse as the two mountain ranges could act as a funnel concentrating the air masses into an even smaller location.
Hope this helps!