Ok...I'll try to make a clear line here
The on-camera flash will fire at full power using lenses without an A contact, regardless of shutter speed less than or equal to 1/180s if in M or X modes. You would need to adjust the aperture and/or shutter speed to control the exposure.
Off camera/Strobist with manual lenses is easy since you have a "controllable" flash. Therefore, you can use M mode on the camera, use an aperture of your choice, then dial in the power of the flash. You will not have any automation whatsoever (good for Strobist things)
And just as an FYI in case this comes up in search results...Cactus Triggers will only work up to 1/160s.