You can do this with K2 extra fields. Create a new textarea field (preferably with the WYSIWYG option enabled to get the SIGPro button) and then insert additional image galleries as you like (one or more).

Finally use this guide to fetch just this extra field wherever you like in your K2 overrides: getk2.org/documentation/tips-a-tricks/display-single-extra-fields-anywhere-in-your-k2-content

Replying...
1. No, because that would require additional fields to upload the images to be used as thumbnails and there goes "Simple..." from the name :)
2. Sorry, I rushed to say the buttons would be loaded when apparently they aren't. Noted to check it out. You can use a textarea field with or without a WYSIWYG editor. Content plugins are parsed either way, so using {gallery}...{/gallery} still works.
3. You can actually (as in the demo). You just need to do so manually (this is a feature to be expanded in SIGPro v4). See the "MANUAL GALLERY SNIPPET SETTING" in the SIGPro docs: www.joomlaworks.net/support/docs/simple-image-gallery-pro

1. It's really overkill, sorry. Not to mention at *least* a few people are bound to use "thumb" by accident at the end of the filename and then there we go... :)
2. Already on my list to resolve for K2 v2.10.4.
3. Great.