Does the greateastern cashflo cc has any processing fee for splitting up the payments into installments and are there a lot of merchants that accept it?

There's no processing fee for splitting up the payments, except for great eastern premium payment (effective October 2020, based on the SMS I received earlier this week)

As long as the merchant accepts mastercard, then they should accept the OCBC Great Eastern Cashflo Card. They will just swipe the card just like any other Card, OCBC is the one splitting the payment in your monthly statements.

The splitting of payments into 3 months (<=$1000) or 6 months (>$1000) is a card feature. Do not confuse this with the interest-free purchase plans (IPP) that merchants offer, thus is a tie-up between the merchants and the banks - if you see a 6/12/24 month IPP that the merchant offers with OCBC, you may use any OCBC credit card.