Well...it seems it was just a battery issue, isn't it?
I mean...as you did the battery mod and placed a new battery inside the unit, no more error codes on startup, everything works fine and it sounds in awesome way?
The seller should have mentioned the battery was dead and needed a replacement, ok...but the unit is actually in very good conditions as he described it.
What I would suggest is to go over and finally enjoy an MP-1 that works fine as you want.
When I spoke to the guy who sold my MP-1, he said it was like new (and it is!) and never mentioned the issue with front/rear input.
When I started testing the unit and I found the issue, I was thinking to contact him back and ask for an explanation.
Then I figured out a possibile answer: "I didn't notice that, sorry...I don't know...I never used the rear input...".
At the end I payed a fair price, the unit was very clean and sounded in awesome way...just had that strange behavior with inputs...
What I decided to do was to register to Adadepot and contact a guy I learned to know by reading about him around: MarshallJMP.
In this way I solved my issue, I found a good place and new friends and I enjoy my MP-1 a lot.
I never contacted back the seller and I'm still happy.
Go over it.
Just remember how many MP-1 you had to buy before you find the right one.
Now you find it and it only had a dead battery.
Furthermore, I don't know how it works there...but here in Italy it's a little messy whit shipments during XMas time.
I also bought my MP-1 10 days before XMas and received it some days after it.
In this case, I would leave the seller alone.