[Nexus 7 (2013) Q&A] xbkingx:
I'm not a dev, but I have some experience rooting the N7 (2012 and 2013).

1) Well, it looks like 6.0 isn't coming to the N7 2012, so there is a good chance support for major versions will be dropped soon (within the next year?). They might continue to release patches and security fixes for 6.x a year or two after that, like they appear to be doing with the N7 2012, but this is all speculation. There are probably number crunchers looking at the sales figures and survey results to gauge interest, and there were some rumors recently of a new N7 possibly in the works. Those will both factor into whether support will continue.

2) You will get a notification, but everything I've seen has indicated that the install will fail (won't brick the tablet, but will just show an error and reboot to its previous state). What I think most people do, including me, is just flash the stock image in 'no wipe' mode and re-root after the update. You can also sideload the OTA update, depending on rooting method (I think, but haven't done this myself). Lastly, if you don't mind reconfiguring all your apps or if you have a good backup scheme, you can just do a fresh install then root. I have never had trouble flashing stock in no-wipe and re-rooting, but I've heard some people have had problems. (It's worth noting that if your bootloader hasn't been unlocked, you will have to wipe the device anyways. The unlocking survives flashing, so it's a 'one and done' thing. You only really need to lock it again if you're sending it in for warranty service.)

3) Wug's toolkit is currently borked for most people (version 2.0.6), so avoid it until the thread in the N7 development subforum has some success stories. Also, rooting in Marshmallow is possible, but seek out instructions specifically for M and use the latest versions of any linked utilities (like SuperSU and TWRP). I used Wug's toolkit 90% of the time for updating and rooting, and when it works, it's a breeze. It might be worth waiting for it to be updated to address the current issues. You can use the previous version to do some basic things and as a launchpad for manual tweaking, but the main, one click and done options will not work - I tried and was greeted with a bootloop due to an out of date SuperSU. But, in version 2.0.5 you can unlock the bootloader and flash the latest stock (haven't tried the U or V versions, but MRA58K worked fine). From there you have to root mostly manually, but there are shortcuts for some of the steps in the Advanced Utilities section.

4) It's always possible to brick a device, but it's very rare. There are 2 kinds of 'bricks'. 'Hard' brick is what most people worry about - something happens that prevents access to even the lowest level of the device. It's as if there was a hardware failure. 'Soft' bricking is what less tech literate people equate to hard bricking because to them the device is useless. In reality, it's a software issue that prevents some early stage of the boot up process. It is almost always recoverable with enough time and research. In basic computer terms, hard bricking is failing to post and soft bricking is failure of the OS to load (basically, though there are nuances). With root, you have access to everything and you're trusting whatever you install that takes advantage of that power to use it wisely. An ad blocker shouldn't need to adjust the CPU voltages. This is rare if you don't intend to overclock or mess with low level hardware tweaks. It is much more likely, but still pretty rare, to soft brick the device, which means you just have to reflash the system image. Worst case is you have to wipe the device and start over, or spend a few hours researching exactly what caused it.

Hope that helps. I'm not an obsessive ROM flasher and my experience has been only with Nexus devices, but these are my impressions.