how to resume GRUB2 upgrade apparently blocked by /boot filesystem type=ext2?

[Tom Roche]

[footnotes follow .sig]

summary: a recent package update/upgrade on a Debian box installed 
kernel-related packages (including GRUB packages), then failed to install GRUB. 
How to recover?

details:

As detailed here[1], I have an otherwise-up-to-date Debian box on which I 
unwisely put filesystem type=ext2 on the `/boot` partition:

    $ mount | grep -e '^/dev/'
    /dev/sda3 on /boot type ext2 ...
    ...

That appears to block upgrade of GRUB2: when I recently ran `sudo apt-get 
dist-upgrade`, it successfully installed several kernel-related packages, but 
then

    Setting up grub-common (2.02~beta2-22+deb8u1) ...
    Setting up grub2-common (2.02~beta2-22+deb8u1) ...
    Setting up grub-pc-bin (2.02~beta2-22+deb8u1) ...
    Setting up grub-pc (2.02~beta2-22+deb8u1) ...
    Installing for i386-pc platform.
    Installation finished. No error reported.
    Installing for i386-pc platform.
    grub-install: warning: File system `ext2' doesn't support embedding.
    grub-install: warning: Embedding is not possible.  GRUB can only be 
installed in this setup by using blocklists.  However, blocklists are 
UNRELIABLE and their use is discouraged..

At about this point, my console went to character-mode-graphics to present a 
dialog with title=`Configuring grub-pc` and body telling me that `GRUB failed 
to install`:

    Do you want to continue anyway? If you do, your computer may not start up 
properly.
    Writing GRUB to boot device failed - continue?

I hit button=No. At this point, I have 3 questions based on 2 assumptions. The 
assumptions are

1. I won't be able to boot my new kernel until I enable whatever does the 
post-package-install actions to complete successfully (and not put up another 
`Configuring grub-pc` dialog).
2. As part of that enablement, I should first update my {`/dev/sda3`, `/boot`} 
from filesystem type=ext2 to something else.

Does that seem reasonable, or am I misunderstanding the situation? Presuming my 
assumptions are correct (and please stop reading now and tell me if they are 
not!), my questions are

1. To what filesystem type should I update my {`/dev/sda3`, `/boot`}? I'm 
guessing ext4; please let me know if there's a better option.

2. What's the {easiest, most reliable, least disruptive} way to convert my 
`/boot`'s filesystem, on Debian? `fstransform`[2]?

3. How to resume the GRUB2 install (after I upgrade `/boot`'s filesystem) on 
Debian? Note that the GRUB2 *packages* (`grub-common`, `grub-pc`, 
`grub-pc-bin`, `grub2-common`) are already installed per both `apt-get` and 
`aptitude`. So is there a way to recreate the package-install post-setting-up 
actions? Or should I remove the packages and reinstall them?

TIA, Tom Roche <address@hidden>

[1]: http://unix.stackexchange.com/a/259712/38638
[2]: https://packages.debian.org/search?keywords=fstransform