1. Jailbreak in Kindle world = adding extra certificate file next to Amazon's. So, jailbreak itself shouldn't have any noticeable effect on device functionality, battery life... and the adding either works or it doesn't. With the exception of jailbreaking via serial cable (where you're playing with bootloader and n00bs can of course do things like erase/format all partitions) chances of bricking a Kindle using user-friendly "copy something to user-visible storage > search for something OR press Update button" variant of jailbreak are next to zero.
Most of the bricking comes post-jailbreak
when users start to poke around system files without knowing what they are doing. Also from what I remember more bricking (of non-jailbreaked Kindles!) was caused in the past by official Amazon bin updates then anything here on MobileRead.
If you read carefully (and understand the instructions)
and make sure that you don't leave Kindle connected to your PC while doing reboots and bin file installation you have nothing to fear.
Situation regarding jailbreak procedure is now little more complex since now you need to use "initial firmware" bin files that aren't model locked (= it will not stop you from trying to install PW2 firmware on Voyage, which will brick your Kindle). That's extra thing you need to be careful about.
2. In the past jaibreaking had zero correlation to Kindle being with SO or without (= jailbreaking itself doesn't remove SO). But newest jaibreak instructions recommend that you usubscribe from SO before starting with the whole downgrading and jailbreaking. I can't remember the reasons for that notice so you would have to read the jailbreak thread... I thing there were some report of soft bricking that were correlated to devices being SO (someting about "lockscreen"/screensaver and ads downloading during downgrade and/or jailbreak ??). I have no idea since my PW3 was jaibroken while on fw 5.6.5 using different jaibreak procedure.
3. & 4. azw3 (aka KF8) is better and newer format (for example you can't embed fonts to old mobi, you can't use Calibre Editor for editing mobi books, nor can you
add soft hyphens).
But you will have to first make one crucial choice that has effects on many thing including Gooodreads integration, and that also dictates the format you must use.
You have to choose what of the 2 is more important to you (can't have both!):
1) Wireless delivery of books to your Kindle AND having your "not-from-Kindle-Store" books archived on Cloud storage associated with your Amazon account AND thus having "last-page-read" , notes and highlights synced between all your Kindle devices and Kindle apps like you have for all books you but on Amazon.
2) Having Goodreads integration , books on Kindle itself NOT treated as personal document but as books.
#1 means using "Send to Kindle" or emailing book to "Personal Documents Service" which accepts only book in old mobi format (to be precise it also accepts "dual mobi" files that contain both "old mobi" and "KF8" portions i 1 file with .mobi extension, but the main thing to know is that it doesn't accept azw3 files). Also sending books this way overwrites ASIN field so for those books you can't have Goodreads integration because ASIN is used to link books to Goodreads releases.
#2 = must sideload books via USB cable. To have them be "Goodreads enabled" you must edit book file (prior to sending it to Kindle using Calibre) to have ASIN value matching that book's ASIN on Kindle Store == you're faking that you have retail ebook from Kindle Store in order to enable all Godreads functionality. For injecting ASIN value you can use
Quality Check Calibre plugin (but you'll need to manually search and add correct ASIN from each book's page on Amazon and add it to "Ids" field of Calibre's metadata window) as stated on Reddit link.
5. Yes, Calibre's "Kindle Collection" on you PC + "LibrarianSync" KUAL extension on Kindle == automation of collection creation and adding books.
Calibre plugin enables you to create collection containing all books that match some criteria of your choice (author, series, genre, tag, same custom column value...) with couple of clicks. The Calibre plugin creates a .json files containing necessary info and then you use LibrarianSynch on Kindle to import that into database that stores on-device collections.
People usually combine collections and Calibre's metadata plugboards to get the level of book organisation they want. Like have a collection for all the books that are part of the series (= made using Calibre + LibrarianSync) and inside have them be sorted in correct reading order (= book title changed using metadata plugboards).
6. Metadata plugboards are applied to file being sent to you device (as part of the "send to device" routine), so you don't have to mangle title or other metadata for Calibre library copy of that file. When creating plugboard rule you specify device and format that plugboard should apply to, and then the rule is applied to all books of that format being sent to that device.
So to make one thing clear, plugboard pattern/template applies to book's metadata not to book's filename on Kindle's storage. You modify that template used for creating filename and directory hierarchy by customizing "device interface" plugin for you device ("save template" field).
Also, Kindle doesn't take directory hierarchy into consideration... as in it can't use it to automatically create collections... so the reasons for choosing particular structure are aesthetic/OCD reasons OR if you plan to generate collections based on directories by using LibrarianSync option.
7.
That Reddit tutorial is ok, but you would probably still need to read things like Calibre guide for template language to be able to adjust it to you needs (I would't want to have series info added in that format "01.00" I like just "01"), and
this thread on MobileRead (also linked on that Reddit tutorial)
and read about how Kindle gets page count for "sideloaded"/"Calibre converted" books.
As for plugins and tweaks those are highly dependent on personal needs. But I'll leave here screenshot with list of 3rd party Calibre plugins I'm using and you can check plugin threads for each of those here on Mobileread (by clicking on "Plugin homepage" link in Calibre window).
As for reasons why jailbreaking is wort it : well in so many words: it enables you to add stuff and add functionality to your Kindle.
Some of it being:
- KOreader. It's excellent reader when you have to read PDF files (I use it mostly for non-fiction books) because of PDF reflow feature. I'm not that lazy to want to read epub with it instead of spending 5s to convert epub to azw3.
- custom fonts
- custom screensaver/ book-cover-as-screensaver (
can't be used on SO Kindle!)
- editing margins
- automation of collection creation
- ability to access diagnostics mode which enabled you to try and fix some soft brick / bootloop problem , or "just" to downgrade firmware version.