The editor is vastly, vastly improved over MMF. No need for spread values, no limits on the naming/number of variables, has sub events, behaviors that are actually maintained and work, event sheet includes, more options for picking objects, the list goes on and on.
C2 has exe export, there's currently a performance problem in the current version on some machines, but it's being worked on and has been significantly improved in the next version which should be out within a week.
That's another benefit of c2. Generally, updates are about weekly. Yeah, that's the norm!
Data is stored with webstorage, you can use arrays, JSON strings, hash tables, whatever you like.
You can definitely put a demo online and have an exe for download.
People can look at the JavaScript in the web browser, but it's so mangled and garbled it makes no difference. No game is secure from hacks anyway, anyone determined enough would hack the game regardless of the method used to create it.
The main drawback isn't much of an issue on desktop, that html5 performance isn't quite as high as native apps, but it's easily fast enough for 99% of games (I made a shooter with hundreds of bullets, lasers and enemies on screen and even on an older amd Athlon 4400+ it manages fine). The main place performance considerations need to be taken into account is on mobile.
C2 is 2d, but there have been some attempts at 3d plugins in the plugins forum, but performance wasn't too good when I tried them.
I was overjoyed when I switched from MMF to construct. There's a free edition for you to try as well, give it a shot!