for 1) if you ask about the part of speech, you are right.「では」 is a particle.
for 2) when you have not made any decision, you usually say that 「決めてないです。」with 「まだ」
which mean is "yet" as this way 「まだ、決めてないです。」
yes, you are right,「 決めていません」is a formal form for 「決めてないです」
Are these ones what you wanted to get ?