I don't seem to be able to set the flag on the entire torrent and all its files, nor on multiple torrents.
For one torrent you can just select all files, then do:
3 Right click on the file
4 Select Priority
5 Scroll to the bottom set the flag
For multiple torrents -
using the Tree View.
1. ctrl+a to select all transfers
2. right arrow key to expand all
3. ctrl+a to select all again
4. right arrow key to expand
5. optionally: repeat "ctrl+a, right arrow key" sequence as long as necessary to expand all subfolders
6. ctrl+a
7. right click > Select Files > Priority > Front and Back First > On
8. to collapse all ctrl+a, left arrow key
You can use text/status filters or categories to select only some transfers, then do 1.-8.
Do you really need to download all files sequentially or front and back first?