[griddle]

If we get some definitions out the way then it would make this discussion so much more fruitful, and save us from finding apparent contradictions between scholars.

When the scholars or even the Sahaba used the term 'bid'a hasana' it only referred to that good action from the Sunnah which has been forgotten, and then re-introduced and reminded for the people. The one who revives it among a people who never knew of he sunnah has 'introduced' it, thus bearing the name 'bid'a' but only in a linguistic sense.
This is also the case for the Hadith of the Prophet (s) when he said whoever introduces a good action will be rewarded for it and those who do it after him. This only refers to that part of the authentic sunnah which the people were unaware of (e.g. making adhan with 15 clauses or 17 or 19) and is introduced to them so they may act upon it, thereby rewarding the 'innovator' (linguistic def) for it.

This is the Bid'a hasana that Scholars like Ibn Hajar condone and praise.

The other scholars such as Mujadid alf thani and Ibn Daqiq are condemning the misunderstanding of this term, namely the idea that it refers to actual shar'i innovations in the deen which have no explicit evidence yet is considered by them to be good... or if a practice wasn't done by the Prophet (Sallallahu 'alayhi wassalam) or his companions or the early generations of Muslims AND it is done without making it a part of the religion i.e. Sunnah, Wajib, Fardh etc. etc. then it is fine provided the practice itself is not against the laws of Shari'ah. This is a misconception, and it is this that these scholars condemned.