@sirhax
Actually your setup looks fine. The default settings for NRF and CC use IRQ from D2 which is EXT0 for Uno and EXT4 for Mega. I use a Nano IO Shield and a Mega Prototype Shield. Both with NRF 2X5 connector to SPI, etc.
I rigged up a Mega running CosaWirelessSender and a Nano running CosaWirelessReceiver to verify. There are a fair amount of configuration ;-). Anyway it works as it should; both ways.
After the latest improvements for low power mode I needed to adjust CC1101::await() to get the correct wait behaviour between transmitted messages. The broadcast message in CosaWirelessSender could go missing.
Try the default setting with IRQ connected to D2.
Cheers!