1. To achieve optimal results, we recommend using an external IMU just like you did with the RTK GPS system. The onboard IMU can be challenging to keep calibrated. We are aware of this issue and are looking into ways to make it perform better.
2. This is normally not a problem. Both the Locator and Receivers are omnidirectional and will transmit/receive signals in all directions. There is one potential issue though; On the BlueROV2, the normal Locator placement is on one of the sides and all the way to the top of the ROV. With this setup and the receivers located deeper than the ROV, the ROV will be in the way of direct acoustic communication when the ROV is located such that the Locator is furthest away from the Receivers. To mitigate this, you can try to lift the receivers all the way up to just below the surface. If you have limited waves this can work just fine.
3. This is actually the correct way to do it. If you are using an external GPS antenna, the Receivers should be measured relative to the antenna. That way the GPS antenna will be the center and the location of the Master-D1 will not matter.
4. In highly reflective environments, we suggest that you reduce the search area as much as possible. Especially in the direction where the reflective area is. If you had a reflective area 40 meters away from the receivers, you should reduce the search area to ~40 meters in that direction. You can also try to expand the distance between the receivers and to keep the receivers at different depths.
If you are able to send us some pictures of your setup as well as the config screens and diagnostic screen in the GUI, we will look into those and see if we can see any potential issues.