Hello.
If you are still thinking on opening them PLEASE DO NOT! The rumors you have been hearing of are correct. Even under controlled conditions it is very difficult to service them. At the factory we had quite a good workshop to repair all kinds of motors. So we thougnt. We knew we had to keep the working area nearly as clean as an operating room or perhaps even cleaner.Also we had to be very careful on how to disassemble it. To make a long story short we ended up with a working motor WITH ONLY ABOUT HALF of it rated power. After many years I still don´t know what we made wrong or what changed.
I don´t know how they do it but there are workshops that specialize in servicing this type of motors. In case of doubt you may find some after some googling.
In reference to the readings you are right. The reading from leads to frame should be infinite. Carbon dust could be an issue if there is too much at the commutator rings. I´d suggest you use an air bottle to clean it. That might help.
By the way check how scratched are the commutator rings as copper particles can also be causing this.
On the other hand I´d suggest you wait until those motors are 100% ok.
In remembering a broken carbon cap might also be causing those faulty readings.
I hope these ideas may help.
Best regards.