1. Spriter is an entirely different tool for an entirely different purpose. Please read the entire thread to get a better understanding.
No, it isn't. It can handle spritesheet-style animation just fine.
2. 90 degree rotations are standard optimizations when packing spritesheets. Think 'Tetris', where you rotate the shapes to make them fit tighter together.
No, it isn't, and it's incredibly inefficient to waste time rotating the images at runtime as that would just create MORE image data in memory, not less. We're not talking about 1990's-style hardware-supported sprite rotation, C2 is based on entirely different set of technologies. The amount of space saved is absolutely minimal both in terms of file size and memory footprint. I've NEVER been in a commercial production setting where this was ever considered a good idea, in games or elsewhere.
3. I took the time to thoroughly explain the problem so that the community and project would benefit. I want Construct 2 to continue to improve, I thinks its a great application. It already supports independent origins with each sprite frame so supporting a third a party application would be pretty easy to implement. This definitely takes priority over less useful new features like Spriter as using a spritesheet is a requirement not an option. Every single Construct 2 user would benefit greatly form this, even Spriter users would benefit.
Your information is entirely incorrect, and for someone who's attempting to make a point on optimization, you should probably find out what Spriter actually allows you to do and what a spritesheet actually is, especially in terms of how C2 handles them on export.
4. Let me reiterate by saying that If spritesheets were properly packed, the benefits would be HUGE. This isn't a small gain, or a good gain, this is a HUGE gain.
Really, there would be hardly gain at all, if any, if you didn't purposely leave a ton of white space around your example images. Even under normal circumstances, without that additional white space, the gains would be next to nothing.
5. It would not be uncommon to see a large reduction in file size, very large memory requirement reduction and FPS boost. All this without lifting a finger. All Construct 2 games would instantly gain that, no additional work, all done automatically for you.
Yes, it'd be pretty uncommon, because there's a point where optimization reaches a level of diminishing returns/pedantry that simply isn't useful on modern hardware, from mobile all the way to desktop. There isn't a single reason there would even be single-digit FPS gain. Your beliefs that it would matter seem to come from either a complete lack of how modern hardware functions across platforms or a background in an entirely different form of development than that which C2 subscribes to.