Mass-effect – Did Zaeed Massani survive, and was he loyal, if I start a new game

Yes. You can abandon your previous relationship with Miranda to pursue Ashley in Mass Effect 3.

As a rule of thumb, you can initiate a romance with characters returning from Mass Effect, who were romance options in that title, and new characters introduced in Mass Effect 3. You cannot initiate a romance with characters introduced in Mass Effect 2, or characters who were introduced as romance options in Mass Effect 2, but you can choose to continue those relationships if you began them in the prior title.

You cannot get rid of either paragon or renegade reputation. The ratio may change, but getting one does not decrease the other (though it may seem that way with a full reputation bar).

That said, I find your claim that one must have 100% x and 0% y reputation to succeed at the last check highly debatable. The reason the introduced reputation was to stop penalizing players for making decisions against their "alignment".