De-Tune an engine for better fuel economy?

You can but that also requires re-propping the vessel and smaller injectors, and re-setting the rack. What you're going to spend to do so will cost a lot more than any fuel costs savings generally. You're better off running them at 1000 rpms (hull speed) and then running them at cruise for 30 mins every 6 hours (or less)

No 1900 rpms, you want to get EGT up to burn out the slober out of the airboxes and turbos. I will usually bring them up to 1300-1400rpms where the turbo's start to turn and you see the slober burning off for a few minutes till the exhaust clears up, then bring them up the rest of the way.

Why are you leaving one running all night long, you don't have a generator to keep the batteries charged? If you have a generator and battery charger and no other need to keep the running I'd shut the mains down. It's best to leave the same engine running and alternate to the other one the next night....oil temp stays up, water temp, etc. Letting them idle all night with no load is not good for them at all.

Shooo, that is really tough on an engine idling like that. In addition to Capt J's good advice, I'd also suggest changing your oil twice as often as you currently do. A lot of garbage building up in there with long idle times.

Running at idle for long hours can shorten the life of sleeves and rings. Less oil getting to the top, less slinging of oil. But, long idle is less damaging than high turbo heat. Separate banks of starting batteries will solve the emergency start issue. I run 2 starting banks on each main. Probably overkill, but I anchor for long periods in very remote places.

I can't imagine any decent boat not having a dedicated bank for each engine and Genset plus a house bank. Set up this way means the odds of a dead boat are nil.

As long as th engines are running at operating temperature, there is no reason not to slow down to hull speed or less if you want to save fuel. Then yes spool them up for a while to clean them up. Better this way than spending lot money detuning them.

Address the underlying issue, not the symptom. Have the right electrical setup to protect against such events as you experienced.