I will ignore the fact that there are 11 time zones in Russia. Perhaps you meant "Moscow."
There are also four time zones in Brazil.
1) She must be getting home by now because it's about 6pm in Russia where they have 6 hours in advance!
When you use AM or PM in a time, you must include the :00. It is six o'clock in the evening or it is 6:00 PM.
They are six hours ahead (rather than "in advance")
2) She might be arriving at her home now, it's 6pm in Russia.
Comma splice: Two complete sentences separated by a comma, when it should be either a semicolon or a full stop/period.
3) I guess she is about to get home by this time once it's 6m in Russia.
...by this time as it's 6:00 PM in Russia.
4) Russia is 6 hours in advance compared to some countries in South America.
Russia is 6 hours earlier than some countries in South America
5) Brazil is at minus 6 hours compared to Russia.
Brazil is six hours later / or behind - whichever it is / than Russia.
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