ince I swapped the battery I have seen an improvement in battery life over the last three days, each being better than the one before. And it seems to have solved the problem. I have not tried the old battery again since, so I cannot confirm the battery is the problem.
After an OTA, you usually have to give it a few days or more to "settle in". For me, battery life is the same if not better, especially when I don't use the phone. I also use a combination of Greenify and SnapDragon BatteryGuru which help as well to control "stray" apps. I'm going to give it a few days to settle down and then I'm going to disable both and then see how Doze works, but so far, it appears to work great. Under stock, unrooted using Greenify and SnapDragon BatteryGuru, my battery would drain about 3% to 5% overnight. Last night, after upgrading to MM, my phone dropped only 1% in battery and the only change was upgrading to MM. I didn't do a factory data reset at all and performance has been just as good as it was before. Only change was it appears battery life on stand-by has improved to what I was getting on custom, debloated ROMs. I like that.
I doubt it killed your battery. If that is an old one, it could be going bad based on what you stated after you switched. But also, like I said, you have to give the system time to calibrate the battery and such.
As for the not automatically connecting, that's a security feature. A lot of what Marshmallow brings to the table is more security fixes. I believe the same goes for the external SD cards. I can no longer get to it using a generic /storage/external_SD mount point. It apparently now mounts it based on whatever serial number it gets when you format it. Since the external SD card is not very secure (any app could read data there), this makes it a bit more difficult to directly get access to it I believe. One option we don't have with the G3 is the ability to reformat the SD card to behave as internal memory. This basically makes the card unreadable except within the phone. Now that's an issue if your phone dies. Then you have no way to read the card and would have to reformat it, so there's good and bad with it. The good being that it protects whatever you have on the card so that only the phone can read it. The bad, as I stated, is if your phone dies, you have no way to get the data off the card at that point. This might show up in a later OTA if we get any more OTAs for this phone. Doubt I would use it though since I only use my external SD card for storing documents, pictures and music. And my documents are nothing with PI in them. Mainly PDFs of books that I like to take along to read if needs be.
MM probably won't be for everyone, but also, the Material design is not just driven by Android. it's driven by trying to make applications that will appear the same, regardless of how you access them, especially when it comes to HTML5 applications. It's not everyone's cup of tea, but it is what it is.
You can downgrade, but it requires knowledge of using TOT or KDZ files and could result in you bricking your phone if you don't know what to do. But, there are tutorials out there on how to do it on other forums. Do a Google search on "Flash Back to Stock - KDZ Method" and you'll get a link to the forum I'm talking about.