imagecache options to choose by user & support for trueSize JCrop option

I can't imagine how this could happen at this end. The imagecache processes are applied on request - the page asks for this version of that image - and gets it.
CHOOSING what to ask for is a job much higher in the chain. Higher even than imagecache, which itself sits above these extensions. There is no UI provided by imagecache_actions, so what you are looking for is an altogether different module job.

I'd suggest you could develop it by making a selectbox or similar as a field on your image nodes (assuming you are using either image.module OR a content type + imagefield dedicated to holding a single image) and THEN, at theme time choosing which derivative option to display when the node is being rendered. All this is about your custom content management UI, and not about the actual technical image transformations that happen in this module.

There is something called imagefield scale & crop. I don't know where in the chain that manages to insert itself, but maybe you can start there. It fills a similar need to what you re asking for. Either way, it's probably closer to the place to find hints than this module.
I'll bounce it over there, see if anyone has an idea.

My client is using Imagefield Crop and he tries to upload & crop EXTREMELY big images (5250x12750 for ex.). I made a patch without trueSize option so far.

Oops, probably this need to be a separate issue. What I want to do is to allow site administrator to choose imagecache presets while configuring the field.