I am not sure if you still need help with this. I am going to explain a short trouble shoot you can do. From what I see, your camera is metering without taking the flash into account. You take a picture, properly metered by the camera, for example your indoor shot posted above, at 1/15 shutter, 3.5 aperture, with an ISO of 800 resulting in a well illuminated picture with no flash; add a flash that is not synced and you get the posted result.
My suggestion is that you set your flash to its default settings; do not adjust anything on the flash unit itself. If it has an auto setting, use that. You need to start at neutral, and then work from there.
As for the camera, use either shutter or aperture priority, that way you let the camera do the math. I personally like shutter priority.
Set your camera on a tripod if you have one, or on a stack of books, table, etc.
We are setting up a control test. Get something to take a photo of, like a stuffed teddy bear, probably not your cat ;)
Now you have something to get consistent results from.
- Picture of subject in either aperture or shutter priority, flash removed.
- Same subject, same settings, flash attached.
The key is not changing any settings.
What are the results?
Are the settings (ISO aperture, shutter) changing when you add the flash? You may need to engage the shutter release button partially for the camera to re-meter with the attached flash.
If your camera is not evaluating with the flash head, then it could be:
You have told your camera to ignore the flash and the flash is operating as if pre-programmed, and in turn is ignoring what the camera is saying to it.
Your flash sync setting is incorrect. Some cameras allow you to do what is called a slow sync, where the flash fires at the very last second before the shutter closes, you would usually get a black line across the bottom of the photo in this case. Or it is fast synced, where it fires way to soon. But I am pretty sure this is not your issue, yours is exposure.
So third, it is that the camera is trying to use your flash as fill, but the flash is doing its own thing, and firing to a preset configuration.
I would look for something in your camera settings that contain the word "fill" in reference to the flash.
If you can tell your camera to use the flash as a "fill" or something to that effect.
Try to only use the flash exposure compensation as a last resort. +- 1/3, 2/3 etc.
Is either the camera or the flash significantly older then the other? You may have communication issues, I worked like this for a while.
If so just know where your flash is at, and be very familiar with it, if you get an under/over exposed, I would change the setting on the flash itself and let the camera meter on its own, you can get some great results this way.
Use the camera to control the ambient light and the flash to control the subject matter, think of them like dimmer switches in a bedroom: shutter speed goes up, lights go down, shutter speed goes down lights come up! :) Then you would use your flash settings to control how illuminated your subject matter is. God Bless.