Are my actions correct in the prayer of combining and shortening (Dhuhr and Asr combined and brought forward, and Maghrib and Isha combined and delayed), and the Witr prayer as a single unit (rak'ah), during a 15-day stay in a place 300 km away, as well as shortening the prayer after returning from the journey?

The ruling we adhere to is that whoever intends to stay in a country for four days or more ceases to be considered a traveler, and it is not permissible for him to shorten or combine prayers. Therefore, your shortening and combining of prayers during that period was not permissible according to the majority of scholars. It is more prudent for you to make up the four-rak'ah prayers that you shortened during that period.

As for the Maghrib and Isha prayers that you missed, you must repent and seek forgiveness. If you missed a prayer while traveling, scholars have differed regarding making up travel prayers while at home: some obligated performing them fully, while others permitted shortening them. The more preponderant opinion is that shortening is permissible.

However, if you prayed it during the time of necessity (before dawn), it must be prayed as four rak'ahs, and it is obligatory to repeat it if it was prayed as two rak'ahs.