I believe the server/host box being greyed out is set in the default.properties file since that's usually an "enterprise" thing where sys admins want to hardcode the server field for users. I'm not sure if you can set that anywhere else, which means you will have to compile it that way.
There are some situations where that box will become "un-greyed", I believe usually this happens if the server field is not reachable (to prevent a broken login loop). There may be other ways it comes un-greyed.
regarding the password field, spark does some "magic" with the saved passwords to make them hard to figure out if you don't know them already.
You'll notice how it's set to password<somerandomcharacters>=<somerandomcharacters>
Just delete the entire line. Next time you start spark, the password field will be empty and it wont let you click the Login button until the password field is populated.
Also, make sure you do not accidentally have "Save password" and/or "Auto login" selected when you make your "neutral" spark.properties file. Both of those settings could give you some trouble if set.
in the spark.properties file, these properties are:
passwordSaved=false
and
autoLoginEnabled=false
both should be set to false.