For something of this magnitude I highly recommend Crystal Space .
The way you want the player/visitor/fan to be able to travel around in a 3d world would be totally insane to complete in Blender game engine.
With Crystal Space you get all the Goodness of Blender 3d, Plus all the real time rendering power of Crystal space. Crystal space has gone as far as to implement many common tool’s into their game engine.
The biggest thing I think you will appreciate is The terrain system, you can have literally thousands of polygons for your foliage in view at any time without lag.
Another feature you mentioned is the ability to go seamlessly indoors and outdoors, CS has what is known as a portal system, so your players can walk up to a cave and see the nice details inside, when they are not looking right at the portal, it is not rendering anything. This saves lots of CPU power.
As far as your visitors being able to launch the client from a web browser, this can be done with pretty much any game engine, you have to just use Active X controls to launch the client. (for windows only)
Another nice thing about Crystal space is the Cross platform capability, in its current state the blender game engine cannot port the sounds to linux or Macintosh without some serious tweaking and re-distributing python libraries (this means your game would have to comply with the libraries licensing) .
To save you time, CS also will automaticaly bake all your “thing” meshes to a light map for you. In the BGE you need to manually do this yourself (not very hard, but it is quite time consuming, and in some cases requires you to make further UV maps for all your meshes onto 1 lightmap texture)
As far as ease of use there is a great plugin for blender that will allow you to create every thing in your world including player models , environments,lights,scripts (XML,and python), and emitters.You then simply hit a button, or hotkey… a few seconds pass, and you will be walking around in your 3d world.All from blender 3d, no other software needed.
Another nice feature is the way CS handles the bumpmaps and paralax shaders, you simply apply the materials and bumpmaps in blender with the CS plugin. Blender game engine requires you to write a GLSL shader
there are a few games that already use an older version of crystal space.
One is called Planeshift, which is open source. This will make it easy to see how they implement the protocol in both the client and the server for a massive multilayer game. (please ignore the graphics, they where defiantly made by novice artists, and should not reflect the power of crystalspace)
Well, before I end up writing a novel here, I should just link you to the CS website:
http://www.crystalspace3d.org/main/Main_Page
Here is the installer page to make it easy to get it up and running:
http://b2cs.delcorp.org/index.php/ArtistInstall
For tech support 24 hours a day, 7 days a week you can come to the IRC channel here:
irc.freenode.net
#crystalspace