Подскажите пожалуйста, как нужно засетапить сцену в блендере, чтобы затем можно было спокойно менять панораму окружения? Как по крайней мере это сделано в файле code_snippets?

Надо настроить окружение в bleneder internal (https://www.blend4web.com/doc/ru/textures.html?highlight=environment%20map#index-9). А ваши hdri надо cконвертировать в cubemap так сделано в change image

Someone can help me to achieve this ?

Hi. You should selected all objects by Shift and with a clamped button Alt make the necessary settings active

Can anyone help with what I'm doing wrong?

Hello.Maybe you forgot apply scale to object.If problem don't decided, please attached you test file

i don't understand how i can bake from equirectangular image

Hi. Here's a video where Mikhail baked cubemap (https://www.youtube.com/watch?v=93bFdDL_UAQ&t=383s) or not understandable specific step?

why are they converging to a point below the waist

This feature is not supported.You can used simple parent to object, may be particle system.It's depending on the task

If you want to use color texture, you should have grayscale picture(mask). This mask should be represented in the alpha channel of the colored texture. For this, split color texture into r, g, b, a in any graphics editor and
put your mask into alpha channel. Then compose your image back. Alpha channel have only png image.
Then you should use node configuration as on the attached picture.
You can use 2 textures: color and grayscale. But this variant will be not optimized.

how can i change the cylinder color?

You can do it this way…Dynamically change colors signed in the picture

last detail, i want the texture to keep original color

It's easy

I want the image opaque, and the material transparent. So we can see the other side of the png through the cylinder.

Hi. I attached a correct example. if I've correctly understood the problem

Hi. I attached an example