Currently in my state the state hospital is going to be closed because they could not do what you are attempting even when mandated by the legislature.
When you have staff that has worked in a system for 20-30 years & has consistently gotten away with not functioning up to par they will resent the hell out of you coming in & attempting to force them to comply
Literally the hospital is going to close because the staff would not accept the need to change and actually follow their own written policies rather than 'doing their way'.
If I had to offer advise I would say "pick your battles", document the need for the specific change carefully, then educate the RN's first, then with them on board educate the nursing staff, explain explain explain, take questions, qive honest answers, be open to suggestions. Even if it is clear that there is only one solution to the problem you will get better results if you let the staff try to solve it and come to that conclusion, then if you tell them to do it your way. When you talk to your experienced RN's and present the problem to them, have your supervisor present. Make it clear that you have the support of those above you in the chain of command. Try to make it clear, without saying it outright, that any attempt to sabotage you will be career suicide.
Oh, and do be really, really, sure that what you think needs to change actually does need it. Psych nursing is process focused not task focused. Letting someone get a good night's sleep may be more important than getting a set of vital signs. The way you listed a set of failed tasks causes me to wonder about your understanding of this. It might be helpful to look at patient outcomes to identify real areas of failure as opposed to missed tasks. Exactly why does an acute psych patient who is observed every 15 mins, and is in no physical distress, need to be awakened for vital signs?