[Android General] Tzul:
To my knowledge, there are no such flags on this tablet (or older MediaTek-based devices I've had).

If you run the "fastboot getvar all" command, the new bootloader includes this output:

(bootloader) warranty: no
(bootloader) unlocked: yes
(bootloader) secure: no

But these values are hardcoded, i.e. they're unchangeable and not the result of testing some flags.

No, it's not blocked. You can install anything you want. If it is able to run on the device, it will run. ;)

You can do that. But only Lenovo has released working "Android builds" for this tablet so far.

There's no trick involved. You normally can't install a new recovery from within Android, because that's a privileged operation requiring root. But you can do it with Fastboot (if the bootloader allows it), and always with the SPFT on MediaTek devices. And "installing" here means "overwriting the existing data in the recovery partition". For more info about the partitions, read this.

The recovery is a "mini OS", separate from Android. It really is an entire OS of its own, albeit not quite as versatile and user-friendly as Android. ;)

I'm not sure what you mean by "custom recover kernels". The recovery has its own copy of the Linux kernel, Android has another. Usually, they're identical, but they don't have to be. On this tablet, things are a bit peculiar, because Android 4 was 32 bit and running on a 32 bit Linux kernel, while Android 5 is 64 bit and running on a 64 bit kernel. In theory, the recovery should be totally independent and be able to run in 32 bit or 64 bit, regardless what Android version is installed, but the bootloader doesn't appear to like that (could be a bug). So, the recovery and Android kernel have to be of the same "bitness", but they can still be different versions.

Anyway, Android is open source, so you could theoretically grab the AOSP Marshmallow source code and build a version for this tablet. But since Android is based on the Linux kernel, you'd also have to build that. Due to the Linux kernel being GPL, vendors are "forced" to release the source code for their version of it. And you can actually download the kernel source from Lenovo. But, it most likely would not work for Marshmallow without modifications/updates - if this kernel source is complete in the first place (I don't know if someone has tried to build it yet).

But the Android and kernel sources aren't even everything. There are a few more parts, like the MediaTek system services, e.g. the one that unpacks the NVRAM partition to /data/nvram on boot. Those services are neither part of the kernel nor of AOSP, thus getting their sources could be problematic... but they are necessary for a fully working system. You could try to just reuse the existing binaries, but then you can't fix any bugs or compatibility issues that might occur with new versions of Android...

Depends on the nature of the driver. Some drivers live in "user space" and are therefore easier to add, others live in "kernel space" and are hard to add.

Since the Xbox360 controller receiver is a USB device, it should be possible to get it to work without kernel modifications...

Again, the recovery and Android are two separate operating systems. If the recovery can read NTFS and exFAT, that doesn't mean Android can. The recovery runs with root privileges (whether Android is rooted or not... again: they're separate), meaning it has full access to all of Android's files. Therefore it can be used to install/update/backup/delete Android and other partitions.

If you want to use an exFAT or NTFS MicroSD card on this tablet, you can use Lurker0's files with my modification. There are also apps such as StickMount and Paragon that add exFAT and NTFS support, but they get loaded "too late" for the MicroSD card during the Android boot process, and are therefore only really useful for USB-OTG storage.

No, AndroidKitchen is a toolbox for modifying certain aspects of existing Android builds, not for creating new ones from scratch.

All Android devices with unlocked bootloaders are "open". Like I said initially: you can install everything you want on this tablet. The problem is it won't run if it's not 100% compatible with the hardware.

Tablets are little "personal computers", yes, but they are quite different from "PCs". The drivers for all the different hardware parts are "baked into" the Linux kernel. That makes sense for these devices, because you can't really modify/upgrade the hardware like on a PC, and because it keeps the system size small. A modular system would be more flexible, but also bring with it new problems.

Depends on when you'll start making one. ;)