I would be sceptical that a battery 11 years old will be in good condition, normally once the warranty has run out i.e. 3- 5 years , replace as a matter of course.
the battery in the boot powers the controls on the car , if it is failing the whole system will malfunction. did you do a full discharge test?
Another quirk of the car is that you cannot raise the revs with the accelerator in park.
I would have thought piggy backing a 12 volt battery on the control battery shouldn't cause problems, but jumping with a plugin type could well cause damage.