a heart attack could have no symptoms at all, a person could behaving a deadly heart attack and not even know it.

a heart attackcould present itself as moderate chest pain, but the heart could be beating perfectly fine, and no other symptoms whatsoever could be present.

a heart attack could happen, and the victim can have a perfectly normal ECG the following day, but still be as likely to die from the heart attack as a person presenting an abnormal ECG the following day.
a young adult can have several heart attacks without knowing it or being properly diagnosed due to their age, and thus have a ticking timebomb in their chest.

yes

(1) Heart attack symptoms vary both in intensity & in their nature, so yes, it is quite true that many mild attacks go by unnoticed. They show up (sometimes) on ECG traces made later.

(2) l personally would not describe my original heart attack pains as ''moderate chest pain''. It was severe, & began as if it were acute heartburn (indigestion).
But l am sure mild attacks might present merely as ''moderate'' pain.. So, again, the answer is a qualified ''Yes'' as regards pain.
But because the heart is muscles (during an attack) cannot pump hard enough,- being deprived of oxygen- , the pulse rate usually rises (and keeps rising) in order to compensate, & this could not be described as the heart ''beating perfectly fine'' . It would be fast, with a high pulse-pressure, (i.e., a 'strong pulse'), unless ur silly doctors had u on beta blockers, restricting the pulse rate. So this answer is ''NO''.

Finally, (3) the answer to the first bit is ''Yes''. The ECG on the day following the heart attack frequently will be apparently normal. But the answer to the second bit is hazy because of the way you've phrased it.
A heart attack survivor, whether presenting with a normal or abnormal ECG afterwards, is at the same risk of another attack. It depends on the severity of the incident, & the speed he can be given treatment.

It is not the abnormal ECG trace that is the killer. The killer is the arterial blockage. What u say in ur Edit is absolutely correct.

Later Edit:
Love Grade 8 .. l am so sorry to hear about ur Dad.. What it actually proves is that ''occasional check-ups'' by which the doctors diagnose ''fine, Normal'' r virtually useless.. Most doctors (sadly) would not recognize heart attack if it jumped up & bit them on the nose. l learned that from, bitter experience.

Sadly, That is Exactly What Happened To My Dad.

He Had Occasional Checkups Proving That His Heart Was Great And Never Had Any Chest Pains. His ECG is Were Normal And No One Ever Thought Anything Awful Would Happen. My Dad Was A Heavier Man Who Seemed To Be Perfectly Healthy. Then One Night He Woke Up With Chest Pains, Vomitting, And Sweating.

He Passed Away One Hour Later In The ER. The Doctors Told Us He Had A Massive Heart Attack And There Was Nothing They Could Have Done. He will Be Gone 2 Yrs. This Coming March And He Was Only 51.