Dual-booting with FileVault activated (OSX 10.11.6)

[haraldunlimited]

Hi, I'm using a Unibody MacBook Pro 13-inch mid-2009 (Core 2 Duo 2,26Ghz) with OSX 10.11.6. I'm trying to set up a OSX-Ubuntu dual-boot. I've done this a few times already, and so far – having partitioned my hard drive and installed rEFInd –, I can successfully launch the Ubuntu live version (with a few pretty standard driver issues). I don't want to install Ubuntu yet though because I would like to encrypt my OSX partition with FileVault, and I have a few questions:

1. Would there be any issues simply activating FileVault with a working dual-boot already set up? I imagine I might not be able to access rEFInd anymore and thus have problems with booting, but I'm not sure.
2. Could I maybe avoid these possible issues by uninstalling rEFInd, merging the partitions (the new one is empty so far), encrypting the disk via FileVault and then setting up dual-boot (i.e., partitioning, installing rEFInd, installing Ubuntu)?

Also, a more or less unrelated question: Support for OSX 10.11 is running out in fall and my MBP doesn't fulfill the requirements for macOS. So it won't be safe to use OSX anymore in a few months. My MBP still works fine though so I'd like to run it with Ubuntu 16.04 so I don't run into security issues from not being able to update. I was wondering though if it would be safe to exclusively use it with Ubuntu – or if my computer might even be vulnerable running Ubuntu because I won't receive any Apple updates anymore, including those that might concern deeper hardware issues and not only OSX issues? Might post it in a new thread, but I don't want to flood the forum with new threads on my first day already... Thank you!