Often, but mostly from ex-USSR. Others include Chinese and sometimes negros (if you see a negro, most likely he speaks bad Russian
Yes, there are such examples.
Usually the Caucasus accent is perceived very negatively. Peoples from the Caucasus have very heavy accent and never can get rid of it,
even if they lived in Russia for a long time. For example, Stalin spoke with a very heavy accent although he spent most of his life in Russia.
15 years ago most of them did. I do not know how it is now, but they generally have very light accent that disappears as the speaker progresses.
It depends. Kyrgiz and Kazakh people usually speak better Russian; Uzbeks, Turkmens, Tajiks usually speak worse.
Rarely.
No
As I already said the worst accent is usually that of Caucasians, especially of South Caucasians. This is not comparable with anything.