Yes he did and because of his reforms the Empire remained stable for centuries to come
Well,
T. Bryce, Warriors of Anatolia. A concise history of the Hittites (London 2019). disagrees with that assessment. Telepinu did not actually stop the cycles of destructive internal conflict that so plagued the empire. Historians thought so for some time, because the
Edict of Telepinu is one of the only good sources on the early history of the empire
, but the Edict actually changed very little. It does mark the transition from Early to Middle Hittite periods, but the realm continued to spiral into internal conflict in this period. In fact, after Telepinu, the empire would be limited to Anatolia until the times of Suppiluliuma I, and the Kaskas even managed to sack Hattusa once.
What ? It fits the definition
No it doesn't. The only thing the Edict really established was a process for finding the new Great King (i.e. primogeniture). This was an innovation, to be certain, but by that logic, most every monarchy above the level of a tribal kingdom held together by brute force would be considered constitutional.
Not a problem. Mursili can vassalize the cities of Syria and Mesopotamia while adopting their local bureaucracy just like many conquerors without bureaucracy
The Hittites were generally incapable of defending their Anatolian territories against Hurrian incursions (in no small part because they also had to contend with Arzawa and Kaska in Anatolia itself). Holding outlying regions against them was not really possible.
And while it may seem strange to speak to the character of a man dead three-and-a-half millennia, Mursili does not appear to have viewed his campaigns as anything but a big looting expedition meant to enrich himself personally, and beautify his capital city.