Media Library: unnecessary galleries created and custom image sizes ignored

Hello !

I am having quite some troubles with the media library and unexpected behaviour, as I will describe later on. If anyone has encountered the same problems and knows a workaround or does have an idea (hooks, filter – I´d rather not use any more plugins) I´d be more than grateful !

Problem one:
I need to create two additional image sizes (by add_image_size) which works fine. But when I crop, resize,… the uploaded image in the media library these sizes (with crop set to true -> fixed width and heigh) are NOT rebuild as well. Probably I´ve overseen a setting – but I doubt that – is there any hook which I have to use to rebuild my custom sizes anew ?

Problem two:
(connected with problem one) I have deleted all wordpress imagesizes
via the intermediate_image_sizes_advanced filter:

function sgr_filter_image_sizes( $sizes) {
	unset( $sizes['medium']);
	unset( $sizes['large']);
	return $sizes;
}
add_filter('intermediate_image_sizes_advanced', 'sgr_filter_image_sizes');

This works as expected when uploading an image – when I crop or resize the image in the media library theses sizes are re-created (which is not prfered)
So prob. 1+2 together:
I registered 2 new sizes, removed all sizes but the thumbnail size, but when I manipulate an image in the media library these settings are completly ignored.

Problem three:
When I upload an image with the drag-and-drop functionality of the media library it generated a gallery – which I do not need or want.
Can I either disable the drag and drop feature or use any hook, filter the block the creation of galleries ?

Hope that someone can help me with these problems !
Thanks,
matthias