Got the exact radio as you for now. Always had larger Connex's etc till I got the 4Runner and just haven't found any good way to mount a full size rig yet.
Anywho the antenna will make or break a radio and the longer the antenna the better. That being said your Midland with the little wil will work just fine but will probably be on the lower range of distance dx and tx, but for trail and highway usage you should be just fine.
Absolutely nothing wrong mag mounts! Don't gotta worry about grounding as the roof takes care of that for you, closer to the middle the better but as long as it's not on the edge your good to go. They have some built in adjustment for tuning by loosening the set screw and raising or lowering the whip in the mount. Sometimes to get the SWR in check you'll actually have to snip the whip itself. Other than that just hook up the SWR meter as normal to test.
As to your coax, I think you'll have enough. I had the same antenna for awhile and ran it the same way you are and it worked out ok. You def don't want too much extra coax and if you do end up with some, don't wind it up. Zig-zag it under the seats/floor mats/whatever. If you do end up needing more coax. Must truckstops or online radio shop should sell double male ended adapters to attach one coax to another. No experience doing this myself so can't comment on any effect it'll have.
Only downside I can say about the mag mount and running it the way your planning and I use, is I noticed after awhile the wind whipping the coax cable against the roof started leaving some scratches. A mag mount will probably always leave a small scratch or two where the base mounts and no big deal, but the coax if left to flap in the breeze will make its mark. To ghetto rig mine I zip-tied mine to roof rack rail to keep it from hitting the roof. Gotta address that better soon.
Also, while the little wil doesn't have the ability to remove the whole coil and put on a weather cap like the 1 or 5K, you can always loosen the set screw, pull the whip, and park in garages. I did this a lot and left it off for long periods of time with no problems.
Overall you should be happy with your setup and be able to tune it to a respectable SWR. Lots of guys run mag mounts with great success, one downside to the whip though that you may hear while off-road is the potential for it to catch something and quite literally "whip" someone.
Hope some of that helps and enjoy your setup!!
- Jeff
