Is seeking the help of jinn in permissible matters an issue open to ijtihad, and is one who deems it permissible with Shariah-compliant controls considered a fasiq or a kafir?

One should not be lenient in declaring people innovators (mubtadi') or disbelievers (kafir), nor should one denigrate those who are keen on upholding the Sunnah.

As for seeking assistance from jinn, Sheikh al-Islam Ibn Taymiyyah permitted it for permissible matters, provided that the one seeking assistance is knowledgeable in Islamic law and is safe from their cunning.

However, the more preponderant opinion is that seeking assistance from them is not permissible, because they are prone to lying and deception.

As for declaring someone who permits seeking assistance from them an innovator, this is not correct, because the issue is a matter of ijtihad (independent reasoning) and is not explicitly stated in the texts. Those who permit it cite the hadith: "Whoever among you can benefit his brother, let him do so," and the hadith: "Allah is in the aid of His servant as long as the servant is in the aid of his brother."

If the jinn is a Muslim, then it is legislated for him to aid his human brother. If he is a disbeliever, then he is like a disbelieving human whose assistance can be sought in permissible matters, but their misleading, deception, and exploitation by charlatans are not to be trusted.