I am a graduate of an accelerated BSN program (12 months) and have a PhD in a different field (Chemistry). No one ever suggested to me that getting a bachelors degree in a new field was "going backward", and I think it's silly to suggest that it would be. How is going on to a completely different field a move "backward"? In fact, when I graduated, a friend asked me if it felt "anticlimactic" to get a bachelors degree when I already had a PhD in another field, and I responded that graduating with my BSN was way more meaningful to me, because it was going to allow me to work in a field that I really wanted to work in.
As for entry level MSN programs, these might be a good choice for you if you know you want to be a nurse practitioner or some other kind of advanced practice nurse. However, it sounds like your interest is in becoming an RN. In that case, an accelerated BSN program may be a better choice for you. If there are no accelerated BSN programs in your area, you might be able to transfer into a traditional 4 year BSN program and complete it in 2-3 years if they give you credit for your previous course work from your previous bachelor's degree. I know someone who did this and completed a traditional BSN program in just 2 years. Talk to the admissions departments in local schools of nursing or look up their requirements online to find out what their prerequisites are. Also, some have a "recency" requirement for prereqs (i.e., they'll only count courses you've taken in the last 5 years, or 7 years, or whatever their limit is), and others don't (the school I went to had no recency requirement and I was able to satisfy prereqs with courses I took as many as 25 years ago). So, find out about the recency requirements too.
Another thing you should know -- you can get an RN with an ADN, but having a BS or even MS in another field plus an ADN is NOT the equivalent of having a BSN. Depending on what area you live in, many employers these days prefer RNs with BSNs, so having the BSN may help you in the job market. There are also some types of RN positions where a BSN is required -- in my state (California) you must have a BSN to be a public health nurse, or a school nurse. So, although you could also apply to ADN programs, there are some advantages to having a BSN.