That sounds almost like a plan that may work lol.. If you want, you can buy it, see if it works and if it doesn't, just pay for shipping back and I'll refund you. LMK
While I can't guarantee it will fix your pproblem, I too had a high idle with the code P0850. The dummy plug you have is actually because the MSP doesnt have a speedo sensor in the transmission, therefore theres no reason to have a plug. Getting a new neutral safety switch cured my problem and...
Your thread says you have an AT transmission? If you do a MSP ECU won't work for you as its from a manual transmission car, turbo as well. If you're looking for an auto ECU, PM me and I can send you pics.