George,

a NW CFI entry consist of a set of 3 files. Each set is independent and refers 
to a single save set:
 ######.rec
 ######.k0
 ######.k1

Knowing this fundamental will (in)directly answer almost all questions:

1. Is there any problem with an index being backed up by group 1 (following 
completion of its save sets) while the index is also being updated by save sets 
from group 2?
>> This can not happen as these are different save sets leading to different 
>> CFI files.

2. Similar to question 1. What happens if two more groups overlap wherein 
they're running against the same client, so they're both updating the same 
index at the same time?
>> See above.

3. What happens if a backup is running/adding entries to the index while 
'nsrim' is pruning the same index?
>> As long as the save set is 'in progress' , the related files are somehow 
>> locked. 
>> And BTW, why should an index be pruned which has not passed the browse time 
>> yet?

4. What happens if an index is being backed up while 'nsrim' is running?
>> Similar to 3. I am pretty sure that the CFI for that client will be locked.

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