Is it possible to order Russian visa support documents 6 months or more in advance of applying to the Russian Consulate for the visa ?

I'm not sure but you have to check at your local consulate.
If comparing to gettin shengen visa for us - we have to do it not earlier than some time before going to country. So I can suppose that Russian consulate can have similar rules.

Yes, you can make an order in advance.
Tourist visa invitation will be valid for more than 6 months.
But it is important that you actually apply to the Consulate within 4 months of your arrival in Russia – for example, trying to apply at the Consulate 6 months in advance is usually a problem.