For comparator usage you need to be in Idle sleep and the comparator itself consumes about 100 uA. Crazy high value for continuous usage in micropower application.
To the "MOSI hack": when MOSI is HIGH the pot is powered, consumes power and may be measured. When it is LOW it consumes no power but cannot be used.
From the nRF point of wiev the pot is simply a pull-down that is easily overridden by ATTiny output - no problem. Morover as long as CSN is HIGH the nRF ignores the MOSI completely and is not affected by MOSI transitions needed to measure the pot.
(IIRC for operation around 3 V you need ATTiny85V. Non-V variant needs higher supply.) EDIT: I was wrong, non-V works down to 2.7 V.