Hi Chris, welcome to the forum.
1. No. Although PB can import MM maps, that's not what you want to do. Many of us would love to call up a specific Thought in a Brain from an external application, but for some reason PB's developers have been very reluctant to allow this to occur. It's not even possible (officially) to call up (activate) a Thought from a link in Notes. PB also can't activate a Thought in another Brain. We live in hope and like to be surprised. 2. Many members in this forum have expressed the wish to be able to create a timeline, as you'll no doubt discover if you search the forum and also UserVoice. I've posted my best solutions to this problem here, but if you want to do it in the Plex, you will probably need Outline View. 3. I'm not sure why you would want to merge them, unless it is to get rid of duplicates. By your description you seem to have created two identical Thoughts, each with duplicate dependent children. Forget about merging. I would suggest to start at the one you want to throw away, click Edit > Crawl Brain, tell PB to crawl childwards the required number of levels and it will add all those Thoughts to Selection. Check the list and then Forget or Delete them.
PB 22.214.171.124 on Windows XP, J-1.6.0_17