Hello~ I may not be in any LotC power/staff/whatever, but thank you for your feedback! Overall, the shamans are currently in the works of a Shamanism clarification project. A few obstacles here and there, but of course it will get done (hopefully soon) nevertheless! Buuuuut, you already knew that since you're a Shaman, hehe. Anyway, let me address your concerns and my standpoint on them.
1. Shamanism decline! Yes, as it always has been since Vailor and now it just continues to sink. I do not have a valid or concrete answer to this issue, but I can give you what I think of the situation. Yes, the lack of teachers is one of the main causes for this problem - no doubt in my mind. I do not think I need to elaborate on that since it is evident and pretty self-explanatory. As a teacher in shamanism, and probably the only current active witch doctor teacher, finding students who my character deems "worthy" is difficult. Might not be an excuse and I do not want it to be seen as such, but my character truly sees many people unfit for shamanism. Speaking of which, if you are interested in witch doctory, do not hesitate to reach out to me IRP and things will go up from there! (I swear I'm nice!)
2. I could not agree more. Dark shamans, although it is somewhat helpful to the magic's growth, should not be able to teach regular shamanism. Yes, they could have the knowledge to teach, however the Spirits would know your attempts and obviously know that someone is a Dark Shaman and who you are teaching. It would be highly unlikely a Spirit would listen/answer/grant power to you if you are being taught by a Dark Shaman - a being that most, if not all, Spirits would have knowledge of being an entity that goes against the Spirits via (did I use that word right?) eating other spirits. The only spirits who would answer their call is probably another Dark Shaman in the Ancestral Realm or malicious Spirits such as Ixli, oh boy especially Ixli (naughty spirit).
Do note, these are just my views and beliefs on the problems! Sorry if they did not make sense, but I am always willing to elaborate and expand upon them. Anyway, you are in Shaman Chat and we are able to reflect and deal with this issue (preferably after the Shaman clarification, pls). Was this help? Probably not, no no. But, meh, why not pitch in my own two cents.