Fun fact, you probably download the file anyway because it's smaller than one of the torrent blocks. That block contains info from a file you do want, so you download the whole block. Your torrent client just puts that file in a different place.

So ultimately it doesn't make a difference, except to show you the file. If you don't actually look at the files that often, I'd leave that file checked just to make it less complicated.

Yes, it's a problem when you're the last seeder. Torrents are shared in blocks of data, not files. When a file is missing, the whole block containing it is not sent. If you delete a tiny file before a big video file, the video will be broken for downloaders because the first block contains the tiny file and the video file's beginning with the index with video metadata, without which the video becomes unplayable.

it shouldn't if you only have 99% of the torrent, you will only share that 99%

it just means they won't be downloading that random useless text file from you and will have to resort to getting it from someone else. you will still be sharing the 99% you have

Generally, no. Some private trackers will give you trouble for it, but it's not a terrible thing to do. It just means there are less sources of those files for other peers. This could lead to someone not getting the full download and being stuck at 99%, if you're the only seeder online. For popular torrents, no harm done at all

It won't cause any problems