Why is IndiGo adding A320NEOs more aggressively than any other airline? Currently there are about 45 NEOs with PW engines and 6E operates 17 of them. These PW engines have notoriously reported many engine issues and 48 of them have been replaced so far. IndiGo itself has faced 6 engine replacements so far.

Usually, we see that when an Aircraft type reports so many problems, airlines usually halt deliveries until the manufacturers fix them. Even with A320Neo, Qatar Airways has cancelled their order for PW NEOs and Lufthansa is deferring the deliveries.

Why is it then IndiGo is adding them so aggressively? It plans to add 4 NEOs in March 2017 only.
It's quite common for the first customer(s) to serve as a proving ground.
Memorandums of understanding (MoU's) are signed that detail the specifics, which in 99% of the cases are not revealed to the public.
If IndiGo was upset, it would have been loud, but it hasn't been so:

IndiGo did not comment when approached by FlightGlobal.
In a statement to FlightGlobal, P&amp;W says that it is supporting GoAir and IndiGo “to assess the situation and minimise any disruption”. However, the engine maker adds that the issues “are still under review so it would be premature to speculate on the cause(s)”.
&quot;We are working closely with our customers and our suppliers in order to address and resolve these issues quickly,&quot; it adds.
— FlightGlobal

Not commenting hints at a non-disclosure agreement (NDA) in place and quite possibly discounts.