Let me address the bugs one at a time:
In order to do it my way I added some different measures cause I kept encountering issues with my down button meter not appearing at start.
Actually, I had that issue, too. In the version I gave you, I believe I fixed it by having the counter check to see how many ticks had gone by, and if the skin had just started, it would enable the volume down button. My working theory is that when the skin just starts up, it has not yet polled Winamp for data and thus has a 0, which would disable the down button. The up button would then need to be pressed to show the down button again. You fixed it in your code by binding not only the hide, but the show to the same measure.
If the winamp volume is adjusted to a value that is off from 10 at times it overshoots the change of buttons to get max and min. So like if you set winamp volume at 240 and increment witht he 10 button . .it might loop over to 5.
Since you put your hide/show functions in a measure that's never called, it's never forcefully updated, so it has to wait for the update that occurs according to the value you have (300 milliseconds). This allows for the bug you mention, because as long as you reset the update period by forcing an update somewhere else, the one with the show/hide functions can't activate. You can see this when you start at 0 (or 255) and go up (or down) twice, then quickly back once. Since the show/hide function hasn't activated yet, the Min (or Max) button is still shown, and it takes it back to where you started. If you give it some time after going up/down twice, it should update the show/hide function, and should hide the Min/Max button.
Well, this is all theory, as I have no idea how Rainmeter runs the code, but I'm fairly certain on most of it. At least, I hope most of it is right ^^;;.
I made a new version of the earlier skin that works properly (as far as I've tested), has Min/Max buttons (idea taken from you ^^), and allows for a variable increment (you need to set two variables in the code). I set all of the hide/show functions in a MeasureVolumeStageOne, which gets the volume value from Winamp, and a MeasureVolumeStageTwo, which calculates MeasureVolumeStageOne+0. I then have all of the other Measures and Meters refer to MeasureVolumeStageTwo, which forcefully updates all of the buttons.
(This only contains the .ini file.)