sharks + planes as formerly seen on Twitter

I haven't looked into it for this company in particular, but some companies let you contact support and prove you're family and then let you in if you're doing so on behalf of a dead relative. I wish I remembered what the policy was called, but maybe that'll work?

As far as the tablet goes, what you are probably running into is "Factory Reset Protection", and if so then getting in that way is probably going to be the more difficult path. Unfortunately, what you would have to do here is identical to what someone who stole the tablet would do, so the "legit" way to get past this would have been to prepare before things got to this point. As it is, you're probably not getting in this way without an exploit (which might be available given the age of the device, "Factory Reset Protection" should probably be in your search attempts).

Getting access to your dad's account might be the "easier" path for this, but it's gonna be tricky for similar reasons. What you'll probably have to do is approach this from the angle of recovering a dead relative's account, so you may have to submit a request for that. Google does take requests regarding the accounts of deceased family members, but like any process that involves review by humans at Google expect this to take a long while and they may reject your request.

(I suppose really the fastest resolution would be if he has a "paper" backup 2FA code stored away somewhere you could find, but I'm guessing he's either not the sort of person to keep something like that or you haven't been able to find it.)

I would have a copy/scan of his death certificate ready if you're trying to prove you're not a malicious actor. Assuming you can actually contact a real human being haha.

Interestingly enough this is exactly why companies like Google and FB have implemented account heirship features so once the owner dies it's a relatively simple process to transfer all authority and data to another account. Although they don't make these features very visible or anything so it's totally understandable it wasn't set up in this case.

Have you tried asking the phone company for temporary access to/forwarding from the number? I don't think they usually reassign used numbers until many moons have passed. It may very well be like trying to extract blood from a stone, but it's probably more likely than getting a useful response out of Google.

first: if he had any other devices that may still be logged in, those might get into the account

second: if there is no need to get in the account, and the device is the only hurdle, you have options.

• the Tegra K1 in the Nexus 9 has the fusee-gelee exploit, same as the Tegra X1 in the Nintendo Switch. this is your last-ditch way in.
• factory reset protection (which is the hurdle you're clearing now) has bypasses on older Android builds (like the one you're trying to break into).

you should look it up on youtube (the guides are either not written down, or too difficult to get a search term for), "frp bypass android 7" or "factory reset protection bypass android 7.1". don't worry too much about what device it is, except samsung, because they're different.

the bypasses mostly revolve around quirky tricks, like:

going to accessibility settings, setting a few options, escaping into a half-broken settings app, going to "about" and then "copyrights" and then clicking on a url to escape to a browser, then browsing through a handful of webpages until something lets you go to the google homepage, searching a keyword, then you download a file, escape to settings, go to a specific few menus and change some settings, get access to the home screen or a launcher, download an apk to set some settings or turn on developer mode/ADB, and then set those settings over a wire from a PC, then log in to a google account, sign it out, reset the device, then the FRP is nullified.

that's just an example from memory, there's probably other ways too.
Here's an example from a different phone for 7.1, but it should work on your device.
https://www.youtube.com/watch?v=y_P1RHz4ArA

this one might also work, if the above one won't (I think that last one needs a phone/modem in the device to work, nexus 9 is a tablet)
https://www.youtube.com/watch?v=denRPu8u4yk