It has been my understanding that all the ענינים חמורים that it says about זרע לבטלה are talking about if someone does a פעולה to be מוציא ז”ל ח”ו, not about a מקרה לילה. Is this correct?

According to Halachah, the Issur of זרע לבטלה doesn’t apply if it happens by chance, מקרה לילה. Nonetheless, the Shulchan Aruch has a section that discusses various matters one should be careful in order that even מקרה לילה should not inadvertently happen. For example sleeping position and how to fight unwanted thoughts that enter one’s mind etc…

Certainly, according to ספרי מוסר, קבלה וחסידות Teshuvah is required for חטאת נעורים which includes מקרה לילה.
It is written in many sources about the need to do some Teshuva or be extra careful in a certain mitzvah.
Therefore, depending on each scenario, there is definitely an idea of Teshuva and tikkun.
See for example Igros Kodesh vol. 20 p. 82, that a Tikun is to be more careful in the mitzvah of Tefillin.

 

Sources:

See the Mafteach Inyanim in Igros Kodesh vol. 20 page 385 for a list of Igros on the topic of חטאת נעורים.

See Shulchan Aruch Even Haezer siman 23.