Earlier quoted context omitted.
I doubt that South Slavic and West/East Slavic were mutually intelligible at 1200 AD. I doubt West and East Slavic were. But inside those geographic groups they probably were (Czech and Polish AFAIR were around that time).
Depends on your standards, too. Even today, any pair of slavic speakers should have a head start in understanding each other. Put them next to each other for a month and they should be talking, at least about basic everyday things.
I was in Crimea for about 2 weeks (in 2012) they split me Russian there. I couldn't understand a word they said. And I didn't learn to understand than for 2 weeks of travel there.
I could understand some words from Ukrainian (I traveled by train from Lviv).
Another example is Croatian, I've been there on vacation and renting a room. I couldn't understand a word they said and didn't learn any.
TlI can understand some Czech (because this is the closest language together with Slovakian to Polish) but that's it.
I wouldn't mix Slavs from different groups together. They evolved separately and are as close as English and German.