Alright Nin, follow me because I'm going to hit on all of these points and work to clarify as best as I can.
1) I believe necroing should be alright. Sometimes a topic is just shoved pages behind, and if it's revived it'll be active again (aside from the reviving post) -- For the record, Pokemon Reborn operates under the believe that posting in a thread that has had no activity in over two months is what counts as necroposting. Many threads will have run the topic of discussion by this point, or the information presented within them will have become outdated and replaced with something new that has its own thread. There are certain exceptions to this, and they are detailed in both "Good" Thread Revival Vs. Necroposting and The Way of the Bump & How to Do So. The tl;dr is that should you be entering a thread with new knowledge that hasn't previously been stated before in there, it's ok to revive the thread with that information; otherwise, should you just be looking for help, it's in your best interest to create a new thread.
2) I know a forum that locked threads when they hit the 30 day mark, and then within a week or two the thread was gone (probably moved to some cache or something, I'm not sure if you can delete threads) -- Many times when a thread officially crosses the threshhold for necroposting, it's already several pages back as is. Coupling this with the well known fact that the moderators are managing their real lives in addition to managing active threads, it's asking quite a bit of them to actively search for threads that have had no activity for two months and lock them in addition to everything else that they do. While an autolock system is an interesting idea, there are the exception threads listed in the answer to number 1 that would suffer from this as well. In addition, many of the subforums here have a "directory", in which a list of threads that are useful to the viewer are listed, as well as threads that you will not be warned for necroposting in.
3) Another forum was extremely organized, and moved threads that were inactive or just old news into this archive section, actually many of the sections had an archive. -- While this could be applicable, the answer to number 2 is still an issue here, as moderators that are already monitoring active threads would have to sift through other threads to find specifically which threads need to be moved. From there, you get into the numerous discussion that would occur amongst the entire auth team, debating the merits of various threads and whether or not said thread should be moved or not. Currently, the page system that we have works as a type of archive as is. Anything on the first page of a subforum is almost certainly safe to post on. The second page is typically where a user should begin to be cautious of what they want to respond to. From the third page onward is typically where you enter the point where responding is almost certainly necroposting. Of course there are exceptions to this, but that's a very general idea as to how this system works.
4) I think locking would be the best preventative measure, and an archive or something would also be a good idea. -- I've kind of addressed this in my previous answers. However, I will bring this up in our next staff meeting for good measure to make sure the other auth are aware of this as a viable idea to help improve forum experience for users in the future, should we find ourselves able to implement such a system.