I located the diagram for the D415 Depth Module. Left-click on the image below to see it in full size
On the diagram, you can see the three Omnivision OV2740 referenced in the text of the data sheet document (the stereo imagers, and one color sensor).
The data sheet indicates that only the D400 module has a filter on the IR imager (IR Cut) and the other module types have none. The color sensor has an IR Cut filter. So your assessment is correct.
Details on ways to get the raw left and right IR can be found in the link below.
Yes, you can mix the streams in different combinations, including RGB and IR, as shown in an image I created in the RealSense Viewer software below.
Yes, you can overlay images from different streams and align them with each other.
You may find the discussion below on accessing the pixels of the RGB and IR streams useful.