Encoding Lossless Audio to E-AC3 5.1

So, I am entertaining the idea of making 5.1 E-AC3 versions of all of my lossless tracks, for compatibility and space saving purposes. I would be to keep any 7.1 tracks, as handbrake can only passthrough those, any 5.1 tracks would be replaced by an E-AC3 encode of the highest quality/bitrate audio track available.

With that said, I have a few of questions that I wanted to run by y'all before I dive headfirst into updating the audio for all my content:

1. I read that, even if you are selecting the lossless track in handbrake, unless you are passing it through, it will default to the lossy core (DTS-HD MA --> DTS core or TrueHD --> AC3) when trying to encode; Is that true; because, if so, I don't want to re-encode a lossy track if I can help it.
2. Am I correct in thinking that 5.1 E-AC3 @ 640kbps sounded transparent to Lossless 5.1 in most situation? For context, I have a Samsung HW-Q990F soundbar system. It's good, but it's not studio/home theater levels of fidelity.
3. Is it better to make an 5.1 E-AC3 version out of the lossless track, or just use its core (i.e. - DTS @ 1536kbps or AC3 @ 640kbps)?
4. This question, I feel like I already know, but I just want to confirm: If there are a 7.1 and 5.1 lossless track, I should encode the 5.1 to E-AC3 to prevent any funny business occurring with the downmix, right?