I have also had a relatively low bird hit rate with the OMD AF using the MZuiko 75-300, and chalked it up to it choosing the wrong thing to focus on. Branches and leaves seem to be an issue, and I assume that is because they often have higher contrast than the bird. I hadn't stumbled on the smaller focus point trick and will try it.
I do have a higher hit rate if I use the AF area select rather than AF lock with a half-press of the shutter and then recompose. Since the arrow keys can be set to do this AF point selection, and the EVF shows what is going on, you don't have to move your eye from the viewfinder to do this.
To answer the question about changing magnification and MF. Once you go to the magnified view, you can change the magnification with the front dial by default (5x-14x).
There is a setting for bringing up the magnified view anytime you touch the focus ring, but I don't like the camera switching to magnified view automatically, since just a slight touch of the lens ring does it.
I also set a button to bring up the magnified view (note that it takes two button hits to actually get this to initiate, not sure why). First hit lights up the green focus square, and then the second gets it into magnified view. After the first time you do this, each touch of the button switches between magnified view and normal.
Also, you could set a button to turn on the "teleconvertor". This gives you instant 2x magnification, and even without peaking, the burst in contrast when the part of the image you are looking at comes into focus is very noticeable. If you need more magnification, hit your magnification preset button, and dial in whatever works. The total magnification is still the same, so if you already have the teleconvertor on, the levels for the magnification function will read 2.5-7x.
I find both the automatic magnification when you touch the lens, and the two Fn button presses to bring up the dedicated magnification function kind of annoying options. The teleconvertor setting, which does not appear to affect the RAW file size, seems to work for me. Not sure if that affects the metering or not (probably), but it does cut the number of autofocus regions in the frame down to just a small grid. They don't seem to become smaller though.
Hope this helps. The instant 2x magnification is usually enough for me to get reasonable MF dialed in, and I don't go beyond the 5x or 7x even when it is critical and I have more time.
I have used focus peaking on the X20 and I am not getting more reliable MF with that than with the magnified view on the OMD. Not sure I would use focus peaking on the OMD if it was an either or option.