I’m not a long time Prescysol EF user, but I bought a kit a few years ago in order to try a pyro developer.
1. I would not worry about oxidation. If I remember correctly I used this method when developing a roll of 135 film and I encountered no problems. Nor have I had any ill effects from oxidation when using Pyrocat later on. I believe that Prescysol EF is a Pyrocatechin based developer and as such not that sensitive to aerial oxidation:
http://unblinkingeye.com/Articles/PCat/pcat.html
2. I tried this developer before I started to use a more thorough film testing regime. That said, I found the suggested developing time to be ok for medium speed films like fp4, exposed in normal contrast light, but a bit on the short side for high speed films like tri-x that had been exposed under similar conditions.
3. With the same disclaimer as above, I got an EI close to box speed with medium speed films but not with high speed films.
4. I only tried the 1+1+100 dilution. But based on my limited knowledge of Pyrocat I am convinced that you can use other dilutions successfully.
5. I have not tried this either, but I would buy some Microphen or the like for use when pushing film.