Nimi

Does 1.2.4 still open fine? If it does, my best guess is some change on your computer preventing recently created/downloaded .exe files from running. (If there was something truly wrong with the installer and program .exe, no one else would be able to run either.)

Are you unzipping the archive version? Do you have any anti-virus software that could be preventing it from running?

From the looks of it, the pack is x512, which is pretty big and demanding on some computers with not enough RAM. (Or an issue on MI's part because it's limited to 2 - 3GB of RAM.) Try using a resource pack that's smaller in size like x256 or x128.

Experiencing lower performance when your animation is playing is normal, it's obviously doing more behind the scenes to get the scene moving. Especially with bending parts, as the program has to regenerate the shapes in the part each time one of the bend angles change. And as far as I know, there's no way to allocate more resources to Mine-imator/GameMaker games & apps.

Donation-wise, yes. Any amount is appreciated though you should keep it mind that donating doesn't promise anything you want out of the program.

They can be mapped, but not mapped correctly. Due to some workarounds we have to make for handling textures in MI, it UV maps differently from models you'd usually find on the internet every since 1.1.0 when the program was updated to GameMaker Studio 2. (Textures must be square for seamless support and to support mip-mapping. It's weird.) Modelbench would have Bedrock entity support if it weren't for this. (I actually experimented with this before I started remaking Modelbench, this status was an example of an imported Bedrock model though other models weren't so lucky with their texture UVing.) And to clear up any confusion, I don't recall ever making a .obj loader and having it work properly. (This status had their vertex data stored in-program, so I used a tool to convert .obj files to scripts.) Also not taking any bribes for features, you're better off keeping your money and learning a better, free software. (Like Blender.)

The ground block will be keyframable in the next update.

Certain models use multiple textures in a single part for different shapes. The Enderman being one of them due to their eyes being separated and thus can't be changed with a single texture. In a case like this, you'll need to use a resource pack since their format supports replacing multiple textures in scenery and models. You won't need every texture Minecraft has in the resource pack since you're using it for a single model. For getting rid of the eye texture, I'd recommend just adding in the Enderman eye texture however being completely transparent and using that pack as the texture. If that's too much of a process, you could also make your own modified Enderman model using Modelbench with eyes as their own body part in the model so you may keyframe their visibility.

Modelbench only supports loading and saving .mimodel files, as it's only built around editing its file format which is similar to Minecraft bedrock entities. OBJ files are not supported nor are JSON block/item models as they can have different textures per block face which is not supported. There are no currently plans to bring support for either to Modelbench, I'd recommend using another tool and a different format(.json) for making block models.

Bloom is an image post-effect and has no relation to what objects are in the scene, only the color of pixels in your rendered image.(Like color correction, vignette, etc.) If you want to put images on top of your render, you're best off doing it in another software so it has no relation to the post-effects Mine-imator is making to your render.

'Bleed light' just makes objects only be affected by casted shadows for lighting, not anything special. Its appearance is based on an artifact in the shadow casting method Mine-imator uses, not much room for heavy customization.(Nor would I like to double down on something this hacky, this feature was originally only intended for foliage.) Implementing proper (Screen-space) Subsurface scattering is most the way to go like with Blender 2.8's Eevee engine for other surfaces.

There won't be anything the new Modelbench can't handle in pre-existing .mimodel files.(Given they're not corrupted or something.)