Not really. IANAL(inguist), but it sort looks like this:

Russian has progressed/evolved more than many of the other Slavic languages (due to more speakers, being a part of an empire etc.). So there are significant differences between Russian and most other Slavic languages (even the closest ones, Ukranian and Belorussian). These differences are everywhere: in word forms, tenses, stress, pronunciation etc.

Many other Slavic languages (especially Balkanian) retain a lot of forms, sounds, meanings that are archaic by Russian standards: anything from Bulgarian `ъ` sound which disappeared from Russian centuries ago, to words like `лепа` (beautiful) in Serbian which hasn't been used in Russian for over a century, to words like Polish `uroda` (beauty) which switched its meaning to mean "ugly" in Russian.

And the list goes on: locative and vocative cases exist in Serbian, haven't existed in Russian for a long time; definiteness of Macedonian and Bulgarian hasn't existed in Russian for a long time, if ever; etc. etc.

So all in all, other Slavic speakers have a higher chance and luck understanding Russian speakers, than Russian speakers other Slavik languages.

No. You'll likely be able to read if it's another Cyrillic language and you'll also pick up individual words. It's similar to an English speaker reading French - they won't understand the meaning but many of the words of latin origin will stand out.

If you understand a little bit of any, you understand a little bit of both. That being said, Russian grammar is hard for a "Yugoslav" speaker, and people from here much rather learn English or German (or both, quite frequently) than Russian.

I'd guestimate at least 10% speaks both English or German relatively fluently, and I personally don't know anyone from here that speaks Russian fluently.

Somewhat intelligible. If the Russian speaker is speaking slowly, one may get 20-30% of the meaning, from the Balkan side. If the people are really trying to communicate, they will probably just use English.

Russian is not a very phonetic language. However, unlike English, the mismatches are somewhat consistent, e.g. an adjective in masculine gender and accusative case ends with -ого (-ogo), which is pronounced (-avo/-ava), but at least it's always pronounced -avo/-ava.