1. U.P.'s assist others to connect to the network yes. Ultrapeer mode should not have any ill effects upon searching. For me I find it is the contrary. I find I seem to get better (more) search results and less spam when acting as an ultrapeer. For example, as a Leaf I tend to get 30-50 search results or less for a particular search term. When in U.P. mode, I tend to get 50-80 or more.
2. I do not think disabling UP will have any effect on searching for many people, but it might help for some who have limited bandwidth. When I was on ADSL-1, I preferred connecting as a Leaf due to my limited bandwidth. Now on ADSL-2, I prefer ultrapeer mode.
3. OOB means out-of-bounds, meaning outside of the regular search horizon that a standard search would be able to reach. So utilising OOB should assist with finding files, most particularly rare files.
4. I am not sure what improvements were made to 4.16 over 4.14. I only started using 4.16 because I misplaced my 4.14 installer when I purchased a new computer. My particular 4.16.6 version was a little buggy, but 4.16.7 would probably be fine. For me at least, even now every second time I open 4.14 it immediately jumps into ultrapeer mode, more often than my LPE does. My LPE usually needs to wait an hour or few hours before shifting into U.P. mode. Toss a coin. One thing you will notice with LW 4.16 or 4.14 is they will connect to earlier versions of LW 4, versions that LW 4.18 was designed not to connect to. LW 5 versions were also designed not to connect to pre-LW 4.14 versions, but LPE 5.6.2 does have the ability to connect to earlier LW 4 versions.