ok Lets break it down one by one.
1) Prior to SEPMs upgrade to 12.1.6 MP2 do I have to break replications or is it fine to upgrade SEPMs one at a time without breaking the replication.
Ans) No, With 12.1.x there is no need to break the replication you can just upgrade the primary server first and then upgrade the secondary sepm
2) Prior to the upgrade free space on the SEPM installation drive should be three times more than the current size of the database i.e if the current size of database is 5 GB the installation drive should have atleast 15 GB free or else installation will fail ?
Ans) Yes you need to have free space that is three times the size of the database, reason being, during upgrade SEPM will copy the database to a temp location and the upgrade the schema and the once succesful it the replace the original DB with the upgraded DB
3) In SEPM 12.1.6 MP2 the maximum number of content revision we can set is 180. Is this the maximum number or we can go higher . If we set content revisions to be 180 then how much disk space would the Interpub folder would consume approximetely ?
I am not sure about the Maximun no of content revesion one can keep. with the release of SEPM 12.1.5 SEPM definition storage has been optimized and it will keep just the delta for all the revision execept for one revision
4) Since the ATM machines could only be upgraded via the Auto Upgrade feature of SEPM becuase of the slow link . What do you think would be the best practice to for the scheduling to upgrade 100 ATMs , it should be distributed to how many days and how the clients would take the upgrade package , all machines will download it at the same time or they will randomly download the upgrade once we add the upgrade package to the group ?
Ans) for 100 ATMs, I would suggest you to go with a 10 ATM at a time and proceed further to next 10, once all the current 10 ATMs is over. you can distribute it over 15 to 20 days and I would suggest you to enable the time frame window accordingly
5) What will be the size of this delta upgrade package for 12.1.6 MP2 that the client would download from SEPM ( 12.1.5 package is availble in installation package on SEPM since it was upgraded from 12.1.5)
I can not give you the exact number but it shoud be arount 12 to 17 MB and I am sue it wouldn't exceed 40 MB.