Mitsubishi Repair Problems? Ask a Mechanic for Answers ASAP
Unfortunately a dealership is going to be your only reliable option here.
The ECU in these cars requires both a VIN programming and key programming. The VIN programming most well equipped shops or lock smiths can handle, however the key programming no independent shops will be able to do (unless they perhaps have an ex-Mitsubishi employee there with a Mitsubishi scan tool), and lock smiths will be highly unlikely to be able to do it. The third-party hardware to program keys that most lock smiths use is typically cloning software that requires an existing key that matches the ECU already or is grey market software that is basically "copies" of the factory scan tool software.... the latter is fine for most manufacturers, however for Mitsubishi this doesn't work due to them using an external intepreter that the copied software can't emulate.
There is hardware/software out there that can do keys for these cars, the cost is just quite high... and given the incredibly small market share that Mitsubishi has, it doesn't make fiscal sense for most lock smiths to invest in it.
What I would do here given the very long tow to a dealership in your instance would be to call your local lock smiths and inquire about their experience with Mitsubishi keys... if they are not direct in affirming their ability, they likely haven't had any experience. In either scenario if they sound confident or not, before committing to anything make sure that they will not charge you if they are unsuccessful. This will at least clear you from having a large bill from the lock smith and nothing to show for it. Far too often i get these cars towed in to my shop after customers have paid literally hundreds of dollars to a lock smith to program a key with no success, only to be told "it has to sit an hour" or other such rubbish so they can get paid and get out of there. There is no such scenario on these cars... they can either program the key or they can not.If you can't get any guarantee of their ability before hand, then i would just buckle down and have it towed to the dealership. 100 miles is a bummer, but not as bad as paying multiple places to try unsuccessfully of course. As well if you or a relative has AAA you may be able to get a discount or even free tow, so that could be worth looking into as well.
If you need further help or clarification on anything just let me know.