I'm not employee of Matrix or Destroyer Studios but I can give you some answers.
1. in your store there a two different starshatter available
What are you fooling? I see only 1 title called Starshatter and that title don't include anything else.
2. starshatter is right now 4.0.0 (demo is 3.9.0) is the game still under developement and people buy beta versions that they later can upgrade to the finsihed on or is it basically finished? if still under development, when will it be finished?
Starshatter is developed by Destroyer Studios which consist of only 1 man: John DiCamillo, also called milo. I don't know how long he works with Starshatter but I hope it will be very long
Right now he is working with patch which should fix some bugs which were missed in beta testing. I assume that updates game either to 4.0.1 or 4.1. I assume after that more bugs are found and another patch is made. If not, John just might start work with sequel of 5.0. At some point Microsoft Flight Simulator had similar practice with version numbering up to 5.x.
3. your game is available only in your store at the moment as far as i checked, will it ever hit the shelfs (normal offline stores)? and when if so?
No unless employees of the shops decide buy games themselves and put to shelves. Hey Matrix, has this ever happened?
4. what are the online capabilities off starshatter (i.e. how many players? (my gaming clan is playing freelancer right now, we're looking for a new game would a server with 50 slots be possible or totally beyond the reach?), can you set up a server, is the online gaming part of it done yet?, etc)
There are no restricted numbers. So in theory if you have infinite hardware capacity and infinite Internet connection, you could have infinite number of players. This is only theory though.
What comes to rest of your questions, Starshatter includes large number of options and settings. There are 3 options for physic model where Standard is most realistic and very much newtonian (not totally). You can't jump to frigate right away but you need get enough high rank first by flying fighters and/or bombers. When you control carrier, you can command all starships of the carrier group much like in Homeworld games, arm fighters and assign missions to them and more. You can't jump to controls of other starship/fighter though, you can control only the carrier. I haven't played multiplayer yet so I don't say a thing about it.