Sorry, that I have to ask again. I just had time to watch the first video with the basketball. It somehow makes sense. But what he demonstrates at the end, isn't it against what you wrote before? If it's long you go with your blade against the direction of the spin and if it is short, you go with the direction of the spin. Or am I just confused?
If you watch carefully, he makes a distinction between left sidespin and right sidespin. For the sake of simplicity, lets restrict us to pendulum and reverse pendulum servers where both are right handed players. In this case when the serves are short pendulum side top/under and you are receiving on your FH, you receive it by touching it on the right side, you can go either with the spin or against the spin both from this position. Seth pech in his video said that going against the spin helps keep it shorter.
I have seen pro players do both. At my level though, I am really not that concerned about keeping it short because players my level don't really know how to push short and I am going to get a long push regardless. So I just keep the paddle there and let the ball come, and adjust the angle according to the vertical spin. If its on BH, I flick everything.
Ma Long prefers short FH push for receives. You can learn a lot by watching his matches.
If you are receiving a short reverse serve on FH, Fang Yinchi says that because you can't bend your wrist to the correct angle, you have only one option which is against the spin. But you can receive it on your BH which is easier. There is another way which is to lightly hit/poke the ball with a flat bat on FH which works if you're not concerned about keeping it short. He also demonstrates this in one of the videos.
For long serves, he always recommends going with the spin. ie. Inside out FH for pendulum, Hook FH for reverse.
I would also say that receiving serves is a difficult skill to master. It is natural to find this stuff confusing. Also because you are left handed, you have to reverse the concept. If you have someone who can help you practice, you can pick each serve one by one and practice it untill you get it.
There are a lot more videos on YT regarding this and visual media is the best way to learn this. You can only explain so much in writing. Also, watching videos is good to get to "know" stuff. But in Table Tennis "knowing" is nothing, you have to be able to "do" it.