1 I am not fimilar with M2000 system /parameter,
however the pwr adm is percentage of Total power allowed for Call Setup in a cell. So by increasing it you would allow more users on the cell and your power rejection will go down.
2 Turned off, SF 8 meaining set it 0 and SF8=1 will allow only 1 SF user in the cell at one time ,The SF 8 max limit is 8.
I would turn off the SF 8 if you have 100s of rab rejection due to power .
3. Yes CPICH will affect the coverge so please do it only for the capacity reasons and make sure that some other cell will cover the lost coverge , meaning do it in good overlap area