Now I’ve had time to check, you are indeed correct. It seems the warning might actually be trying to tell you to use
fsck.mode=skip to replace what might be intetended by the
fastboot command in cmdline.txt (like for like) as fastboot is not supported in Raspbian.
I had thought it was telling you to use fsck to repair the disk (hence my suggestion) and that the “pass” was perhaps a non-interactive mode, but no that is not the case.
Whilst the fastboot is indeed part of the emonSD, it serves no purpose and if the cmdline.txt contained something like
fsck.repair=yes as does the standard Raspbian image, your issue would probably not of occurred as it would have self-healed.
While it may be necessary for some hdd’s to require the bootup to be slowed down, I’ve never needed to use a delay for any of my hdd’s. I would imagine if the hdd needed more time then other things way before just the apache2 service would fail first. besides the apache2 service is started a second time in rc.local so even if it failed once it would start second time around.
It is far more likely that your issue arises from a change in ongoing access speed of using the hdd instead of the sdcard. In the rc.local file there are some delays (as you’ve noticed) that have been altered more than once over the years with new or changes to the images. This alters the speed that the listed services (which includes apache2) are started for the last time, so it is here that your service is failing to start, not in the bootup. Not because of the initial starting on hdd, but because restarting of the services in this manual manner is not tested with your HW setup, so the times probably need tweaking.
Ordinarily systemctl manages all this but the emonSD undermines systemctl, which is rather ironic really considering how apparently important it is to change the whole project to use systemctl and drop perfectly good systemV stuff.
Hopefully all logs since boot are retained whilst running, so if you reboot and the service isn’t running, the error should still be there.
 Actually, I just walked away from the pc and recalled you have moved the log folder back to disk (out of RAM) IIRC. That might just be the reason. Creating all those folders and files, then changing permissions and ownerships in rc.local may take a tad longer when written to disk rather than held in RAM, and that might mean the timings (delays in rc.local) need tweaking.