LW 5 will periodically re-search all the incomplete files. I cannot remember how often this is (perhaps once every 30 mins or once an hour.) It will attempt to reconnect with the hosts it was before. Of course no guarantee it can achieve this if either (a) such hosts are not online, or (b) such hosts are busy & have no available upload slots. If the case is (b) it may sit in 'Waiting' position. Edit: (c) if LW simply cannot find the original hosts because they are not in search range.

Best approach is to personally re-search the file using the search tool. Do this periodically such as every 20-30 minutes. Then after 5 minutes, right-click the search tab & choose 'Get More Results'. Both of these work a charm for me getting more search results.
People come & go from the network all the time. Perhaps the person(s) you were downloading from are not online every day or LW cannot locate them. Also, I'm guessing LW attempts to find the original ip addresses it was downloading from. Many people these days have dynamic ip addresses which makes it difficult for LW. My dynamic ip flies all over the place lol.

Just last night, I had a few reasonably old incomplete files automatically start up via one of LW's automated searches. These incomplete files were at least a month old, maybe older. I consider this pure luck though. I mean these incomplete files were right at the bottom of my downloads window out of just over 100 incomplete downloads. Likewise I had a couple others start downloading mid-way down. I didn't see them downloading initially because I have my downloads view option as most recently added at top of list. I use the Classic view btw. I never did like the very basic default LW 5 downloads window view. Edit: And just today, I opened LW, did no searches, after an hour I had a download finish. Another is still downloading, if only slowly. In such cases it would be wise for me to re-search the topic to see if I can find more hosts for the file.

Yes you are correct, with LW 5 it will delete any file you Cancel. Unlike LW 4 which would simply keep them in the LW incomplete folder regardless.

Do as I suggested above. The longer you are connected to the gnutella network, usually the better LW behaves with searches etc. Whilst i mentioned people come & go from the network all the time, also your LW changes ultrapeers it's connected to periodically. Thus, it will be searching through a different range of people periodically. Even searching a similar term to what you are after can sometimes help discover users who may in fact have the incomplete file you are wishing to restart.

Another tip, if you choose a file to download & it goes into waiting mode. Wait a minute, then Pause it. Then right-click it & choose continue. Sometimes LW is slow to push to start to download when a download slot from the uploader becomes available.