über with umlaut means "above".
The use of "über-" or "uber-" as a prefix to mean "super, great, special" seems to be mostly an American thing, though there are German words such as übereifrig "overeager" or übervorsichtig "overly careful".
uber without an umlaut means nothing in German.
Would a key bowl (like what some people keep by their door) be called a Schlüsselschüssel?
Yes. And it would be feminine because Schüssel is.
Or have I missed the rules on how to form compound words?
That's actually tricky in the general case, because sometimes we add an extra sound between two components ( https://de.wikipedia.org/wiki/Fugenlaut ) and sometimes we don't -- and if we do, it might be -e- or -s- or -n- or something else.
die Orange + der Saft = der Orange-n-saft but die Kirsche + der Saft = der Kirsch-saft (not der Kirschensaft or der Kirschesaft)
Sometimes German speakers even disagree about which one to use.
And sometimes it makes a difference whether or not you use one: a Gasthaus is not the same thing as a Gästehaus, for example.
The ü in Schüssel is short. Ü can be either long and short, depending on the word. Wherever it precedes a doubled consonant like ss, tt, etc., it's short. When it precedes ß it's long. If you Google around for audio guides to short versus long ü, you'll find examples of the difference.
(Edit: This is according to the spelling reform of the 1990s, which established firm rules that words had to be spelled with ss after short vowels and ß after long vowels. Before then it was less regular, but since the "neue Rechtschreibung" was the only one I learned, I can't tell you just how irregular it was.)
- has must be hast
- Schlussel must be Schlüssel with umlaut (if you can't type that, then use Schluessel rather than simply leaving out the dots)
- eine (feminine) must be einen (masculine accusative) -- Schüssel (bowl) is feminine but Schlüssel (key) is masculine, so the article has to change
"You have a key" would thus be Du hast einen Schlüssel.