The ConnectFix installers install a fresh list of hosts to connect to. At least a percentage of them are reliable hosts and will be online (whether they have spare slots to connect to is the question.) But in theory, you only need to connect to a single host. If the host does as it should, it should pass information to your LW about hosts it has detected as being online during its session. Hopefully you connect to some of these hosts. The connectfix installer replaces any older connection file you have. I collate the host data myself from checking connection windows of LW or Phex and seeing which ones connect the longest and also have static addresses.

I do not know why disabling TLS should help with connecting, but seems to work for some people's systems or versions of LW. TLS is supposed to help provide more secure file-transfers. By secure I mean I think they are semi-encrypted.
I do not know much about the Mojito DHT. According to Wikipedia, "The DHT is replacing search by SHA-1, when locating alternate sources for a known file or looking for push-proxies."
Running as Ultrapeer means you help others connect to the network. With LW you will connect to 32 other peers and 40 leafs. There is a small over-head of bandwidth running as UP. Approximately 10 KB/s or a little less. Some persons prefer to connect purely as leafs so they can maintain all the bandwidth.

LW 4.18 I will not say I especially liked. I preferred LW 4.16 or 4.14 with 4.14 being my favorite. LW 4.18 was the first version to change the location of the downloads.dat file which keeps a record of all your incomplete downloads. Previously this file was found in your incomplete folder.

Most people found that when downgrading from LW 5, LW 4.16 or earlier were more successful at continuing previous incomplete downloads. LW 4.18 on the other hand seems to almost ignore them and will not recognise they are incomplete files when you do searches (ie: same file shows in search results but LW 4.18 will not indicate that you are already downloading that file.)

If you wish to try 4.16 then see http://www.gnutellaforums.com/downlo...downloads.html which will show the process to move the downloads file to the incomplete folder. And then how to resume the downloads.

As for resuming incomplete downloads in general, re-searching the original file topic, and doing variations on this. Search results are usually better the longer your LW has been running. One thing you can try with LW 4 is pausing downloads then resuming. Or using the technique used in the link above. This is something you cannot do with LW 5.