Ron Kramer (et al),
The Astrophysics Experimental driver is intended to be used with the newer GTOCP(X) mount controllers, specifically the GTOCP3 & GTOCP4 with version V or later firmware. If you have one of these controllers with the proper level of firmware then you should be using this driver to take advantage of some of the features in these newer controllers. That being said, it is still labeled experimental as it is felt that more testing is needed. I have used this driver on my AP900 with GTOCP3 controller quite successfully for many months. Michael Fulbright did a great deal of work implementing this driver. I worked with him and Jasem testing the driver over a period of weeks during the main development phase. There is a good deal of documentation for the AP mounts here:
www.indilib.org/devices/telescopes/astrophysics.html
I highly recommend you spend some time reading this information which will help in setting up and using your Astrophysics mount with INDI.
About guiding the mount please consider the following:
1. Astrophysics recommends using a guide rate of 1 for these mounts. This applies to using pulse guiding or the ST4 connection. This rate is set in the INDI control panel (see the documents linked above).
2. If using Pulse Guiding then make sure to turn on the "Use Pulse Cmd" in the Main Control tab of the AstroPhysics Experimental driver from the INDI Control Panel. Then you need to choose "AstroPhysics Experimental" in the via selection of the Guide Module in EKOS.
3. If using ST4 Guiding then make sure to turn off the "Use Pulse Cmd" in the Main Control tab of the AstroPhysics Experimental driver from the INDI Control Panel. Then you need to choose the CCD or CMOS camera in the via selection of the Guide Module in EKOS (For example - ZWO ASI178 or whatever camera you use for guiding) so the guiding will then be done from the ST4 port of the guide camera.
4. While using the Pulse Guide option further consideration need be given the pulse length as set up in the options (click the "Options ..." button on the lower right corner) in the Guide Module of EKOS. (Following is a simplified explanation) Any pulse length of 1000 (ms) or longer is handled with a different move command in the driver rather than the standard pulse command due to a limitation on the GTOCP3 controller. Michael had this working well and then had to fix this again in recent weeks due to a regression in the code. I have tested his recent fix and it seems to be working properly again.
I use a pulse length of 900 (the default unit is ms) and I usually only do 4 or 5 iterations and have found that this works well for the calibration of the mount while using either the ST4 connection or the Pulse Guide function. I have been using KStars/Ekos/INDI with my kit for 2+ years and in recent months run several all night "automated" imaging runs with good success. I understand the frustration of software bugs or user error in learning a new set of tools and encourage you to keep at it as I am sure it will come together for you.