1. Doesn't serve any purpose for home brewing. Just get what you can.
2. Sometimes you can get a little air pocket, but it doesn't affect the filtering in anyway... so up or down, its your choice.
3. I don't believe it would be possible to gravity feed these. I only single stage filter, but even then I have had to bump up the pressure a bit as the filter starts to clog. My process for filtering is to rack from the fermenter into a keg, then transfer from beer out - through filter - into beer out of the serving keg (fills from bottom that way).