Eagular, been following this thread. I believe your config, at least on the EX side is improper. On the EX4300 none of the interfaces should be members of an AE, they should all be single links. The Reth config on the SRXs takes care of making sure traffic goes across the proper links. This does assume no AE config on the SRX side, which from the config snipit you send appears to be the case. I would start there, and then see where you are at.
Refer to this doc: http://kb.juniper.net/InfoCenter/index?page=content&id=KB22474&actp=search&smlogin=true
Forget about the lacp config part, not needed. You have configured like the top portion which is incorrect. Take out the EX ae1 portion of the example and it would now be correct. And since no AE, no LACP. Forget about the rest of the article, as that discuss how to properly configure if you actually want multiple links active in an AE between EX and SRX. A diagram with this article may have helped. Just FYI if you want AEs on the SRX to be part of your Reth 0 and Reth 1, you would actually need 4 x AEs on the EX side.
Questions, just respond to this thread. I have not yet found good documentation, IMHO, regarding how to configure this fairly common connectivity between SRX cluster and EX VC, at least not one with a diagram that explains things well.
Good luck.