Creating an FLV movie with a transparent background under Ubuntu?

Question

linux

Tell me, please, by what means can I implement the following:
There is a video clip in MOV format. It was taken with a DSLR in portrait position.
It is necessary to turn it, cut out a piece from it with a speaking and slightly moving person. Next, create a mask by cutting off everything superfluous (or remove the chroma key background, if shot on it). Everything outside the mask should become transparent. The final dimensions (aspect ratio) of the video on the sides are not fundamental, but ideally, it should be exactly the same size as the “semantic” part. The video is needed to be placed on top of other content. After such processing, it needs to be rendered to FLV with an alpha channel.
Is it possible to implement this under Ubuntu? I tried Cinelerra, OpenShot Video Editor, Avidemux. In Cinerella, I almost achieved the result - a video with a black background is rendered. But it is not possible to go further and still make a translucent FLV, Cinelerra does not support such rendering.
I assume that software like Adobe After Effects will solve my problem, but its cost does not match my capabilities yet. Perhaps there are some other, simpler or, say, direct ways to solve my problem?