If a case is currently in the appellate court being appealed by the Plaintiff, can the Plaintiff suddenly submit an amended complaint in the lower court?

This question should be directed to the area of civil procedure; but seems to me, the amended complaint would be subject to a motion to strike or demurrer with prejudice.

The answer to your question is generally, "no."  Appeals most often happen after a case is completed - either with a jury verdict or judgment by a judge (summary judgment or dismissal).  After that, an amended complaint doesn't revive the case.  Even if someone appeals during a case (an "interlocutory appeal"), the case is usually "stayed" while on appeal and nothing can be done until the appeal is complete.