Efiled a return by mistake. it was not even started!

Join the club. Many of us have done it. There is absolutely nothing Lacerte can do about it. Once submitted IT IS GONE, GONE, GONE!

Now line everything up to do the amended return. Begin the amended BEFORE making any new data entries. That way the "as filed" is filled in for you.

At least now we can e-file amended returns, much better than the dark ages when we had to do paper.

Was it actually accepted?  As you say, all of the errors should have caused a rejection.

Under Settings, Options, Tax Return, Tax Return Options, change Computer Date as PIN Signature Date to No. You don't need a date to e-file an extension; without a manually-entered signature date the return won't e-file. You'll want to change that for all your modules.

There's no e-file rule that says an entity return with no revenue isn't e-fileable; my guess is that if you look at Forms, the balance sheet will be blank (because Lacerte thinks it wasn't required) so being out of balance isn't an issue, either.