convert whole drive?

Yes, definitely possible. I do it all the time with a program called Sound Grinder, which I like. I'm sure there are plenty of others out there that are also good.

Yes, it definitely converts from FLAC to Apple Lossless.

Might be Mac only, sorry about that.

dBpoweramp.... Claims to do batch conversion. 

I don't touch windows when avoidable so I can't exactly test it though.

dBpoweramp will do it but is likely to throw the whole mess into one folder for you to sort for days....

I want to do the same in reverse, i.e. ALAC -> FLAC, but it scares me frankly, one album at a time

The dBpoweramp batch converter will do it.  It does ALAC both converting from and to ALAC.

The batch converter will allow you to set a directory path based on tags in the original files.  So all of your converted files will not end up in just one directory and a giant mess.  It will actually organize things as it converts.  But that requires that the originals are all tagged properly.  And you'll have to experiment to make sure it does the right thing for multiple artist albums (I prefer multiple artist albums to be grouped together as an album rather than separated into individual artists and doing that will require using the [album artist] tag and the files will need to be tagged correctly with that tag).

I have the paid for version of dBpoweramp (it really is good enough to pay for).  The free version has a 21 day demo period which will be long enough to allow you to convert your entire drive.  If you are doing a large drive expect the conversion to potentially take a couple of days.

Good luck with it.  I guess I'll have to finally pay for mine so I can use it properly as I've only been using it for ripping since the trial period elapsed.