Thanks for your question
When an individual leaves the UK to live and work abroad, (which must be for more than a full tax year) they can spend no more than 90 days visiting the UK, none of which can be for work purposes
However - as you will be working a 5 week on and a 5 week off - you may not be treated as actually having left the UK due to
1) Returning to the UK in between each 5 week work occasion
2) I assume retaining a home, and possibly having family remain in the UK
3) Be paid by a UK employer
4) Exceed more than 90 days each year in the UK for non work purposes (although maybe some of these will be work purposes if you are an office based employee)
So in answer to your questions (whether treated as resident or not resident)
1) If the company pay your Russian tax obligations, then you cannot have credit for these to offset against your UK tax, but if these are deducted from your pay, then yes you can have them offset against your UK tax position
2) You will remain liable to National Insurance if you are treated as not resident for up to 52 weeks, but if HMRC determine that you remain resident for UK tax purposes, then you remain liable to national Insurance for the duration of the employment with them.
Do feel free to ask follow up questions, from any of the answer I have given.