How to disable auto Sleep Mode (including its schedule) and only use manual Sleep Mode?

You can hold down the upper left button that brings up the function wheel and there will be an icon for sleep mode. You cna click to disable it.

I did this, but it still goes into sleep mode

I did the 1 minute sleep mode hack for a while and shifted the minute around (late morning, midday, afternoon) but it always resulted in even worse sleep tracking than I usually get. My sleep is apparently hard to track but the 1 minute sleep mode sometime during the day and manual sleep mode at night resulted in Garmin thinking I slept the whole day or some hours during the day that removed the perfectly fine data from the night before. Sometimes the wrong sleep time included recorded activities with HRs of 160 or more. And pretty much always the wrong sleep detection resulted in bad sleep scores that affect the training readiness and that can't even be corrected manually most of the time. The only thing you can manually correct is the sleep time and that does sometimes update the sleep score and almost never the training readiness, maybe if you catch it very fast or I don't know. And sometimes it's more than once in a day that the sleep is wrong and you have to correct it again.  It's really stupidly implemented. If they did any kind of sanity checks they would never produce such useless data.

Seems then that what you have to do if work shifts where work hours changes that you need to adapt your sleep schedule in watch regularly to match your actually bed times. 

A great thing would probably be if Garmin could add option to enable/disable sleep windows and if disabled you have to manually enter sleep mode. And when sleep window disabled then watch only detects sleep in case you entered sleep mode manually 

That would be great. And I'd guess pretty easy to implement. I have no problem pressing two buttons (you can assign a shortcut to sleepmode on/off) when I go to bed and again when I get up and inside that time the watch can try to detect the sleep and it might even get it right with that help. And even if not, it's not going to be too wrong, at least I was in bed trying to sleep.

And maybe the 1 minute trick works for people who don't have times during the day where the HR and HRV and movement are similar to those at night. Sleep tracking seems to work well for most people but if it doesn't work too reliably to start with because your resting HR is more or less the same during the day when at rest as during the night while asleep then the watch does really stupid things when the power sleep mode activation (for the 1 minute) is not when you actually go to sleep. It seems to try really hard to find the falling asleep moment around the activation of the sleep mode +-2h maybe. And it completely ignores stuff that happens during the time it thinks you're asleep. Sure it says you were in an awake state if you're recording an activity on the bike at 25kph with a HR of 150 but it still lumps that into the sleep time if it 'detected' a falling asleep an hour before you went out because you lay on the couch watching a movie at that time, with resting HR, not much stress and no movement. And then it gives you a really bad sleep score because and hour in front of the TV plus 2h on the bike did not make for a good nights rest. Once I had a connectIQ walk a recorded bike ride and an elevated HR because of a hot bath all inside a sleep window during the day. No idea why it never detected a 'waking up' during all of that plus the hours in between resting while awake.
And you only see the wrong sleep detection on the phone when you look at the sleep times and sleep score, something you do in the morning but not necessarily all the time during the day. The watch looks normal while it detects sleep without the sleep mode on and you can use it normally and it still thinks you're asleep.

One really bad example. That's two connect screenshots of the same day. I got up at about 8:45, recharged the watch around 13:00, then did some indoor cycling on the trainer, then a walk through the park and a hot bath to relax the muscles after the hard ride. The whole day I apparently dreamed until I started the ride then I was at least awake but not awake enough to end the sleep detection ;)

That is bizarre, I have numerous days where my HR would correlate well with yours but never had sleep continuing after I got out of bed.