Hi Lismi,
1. The AD9122 core is just an interface core, which ease the access to the AD9122 device, so you should use this core either way. The question is where do you want to implement your modulation algorithm, in software or hardware. If in software you don't need to modify the hdl part of the system, if on hardware, probably the best choose would be to put your modulation core before the AD9122 core.
2. The ADL5375 is a broadband modulator. You need to do the data modulation in software or in hardware (hdl), then feed the DAC (AD9122), with the result.
Thanks,
Istvan