Mine is Local Send which is a FOSS alternative similar to air drop that works across a variety of devices.
You can convertí the files in another playable format with Handbrake, probably you Won’t need Change the codecs of the files only the container and the conversión will be fast than reencoding all
You can, but it’s a lot of number crunching time to convert a bunch of files like that, as opposed to just using a different player.
Yes, this would be an option (that I did not think of). But I assume that it would be easier to download the same file in another file format, as there will be probably an improvement regarding the video resolution (480p versus 1080p or higher).