Can officers be demoted?

If it happens... it's very rare. I would also assume you'd be less likely to see it these days than earlier times perhaps. My grandfather somehow remained a second lieutenant despite regular re-assignment to new aircraft and more dangerous missions. I suspect it was because he was a pilot before the War, had no college degree, and reputedly liked to get into fights in town. My guess is that he was a "good stick" and they were short of people... or he'd have been cashiered.

Yes, but it requires a formal reduction board and approval at the highest levels, I saw it once, and it was accompanied by resignation from the service

Under certain circumstances, a Warrant Officer may be returned to their last NCO rank. It's usually in conjunction with non selection to CW2.

I don't know about demotions, but I am pretty sure that promotions will stop. When I was stationed in West Germany back in the 1980's there was a new 2nd LT who had been in country about two days. He decided to rent a car and tour the German countryside. He ended up about an hour from the nearest US Instantiation and got drunk, busted up a Guesthaus, and was picked up by the local polizie. Since he was assigned to the 8th Inf. Div, MP's from Bad Kreuznach were dispatched to pick him up. When the two MP's arrived (2 Spec. 4's) they were met by a Polizie officer who spoke fluent English. The mission was to escort the Lieutenant to BK and turn him over to the Div. Staff Duty Officer. He was in the holding cell, screaming things like "We Won....Twice" and speaking German like the way comics did on TV. The senior E-4 Identified himself, and informed the Lieutenant that they were giving him a ride to Div. HQ. The LT, swung around and kicked the MP in the family jewels, and was trounced upon by 5 Polizie officers. He was handcuffed and the Jr. E-4, placed the LT under arrest for disgracing the uniform. (only officers are allowed to arrest officers unless the circumstances like this arise) When the MP's got back into radio range with the MP Station and told the desk Sergeant what had transpired, there was a long pause on the radio. Sarge just said to maintain radio silence, get back to the station ASAP. Once they arrived, the Lieutenant was hustled into a holding cell, and the MP's were told to write very detailed reports. While writing the report, a Major arrived (Div. Staff Duty Officer) and questioned the two E-4's about the issue. After hearing what they had to say, and reading the reports, and the translated reports from the Polizie, and damage estimate to the Guesthaus, and the assault on the one MP, The Major stated that the MP's did just the right thing. The Lieutenant, now more sober was released to the custody of the Major. Turns out the LT was supposed to meet the Division 2 star with all the other newly arrived Officers for cocktails at the O club the next night. What DID happen was the Lieutenant was given orders to be reassigned to a weather station North of Juneau, Alaska, and to be shipped out on the next available flight out of Ramstain AFB. So he was never heard from again, I suspect that his carrier in the military was very short lived.

There are reduction boards, the same for more senior NCOs. But more often than not, an officer with that kind of misconduct is punished and separated from the service, so you rarely see them stay in after that kind of action.

I can't find the specific regulation, but I do know that they can be reduced in rank. This is usually followed by some serious transgression on their part. A few years ago, there was a General Officer that was in command of AFRICOM (LTG William E. Ward). He was a 4 Star. Got popped for using his Govt CC in inappropriate ways plus a few other things. He lost one of his Stars, then was retired. I've also read about Colonels that got popped for something and was reduced/retired at their last rank where they served Honorably. Usually LTC or MAJ (depending) and are then retired or removed from the Service.

I am sure they can. Regardless of rank enlisted or officer, no one is exempt from UCMJ action and reduction in rank. Just depends on the severity.

I knew of three that had to take a bust in rank , they only lost one rank.

My input is almost 30 years old.

Before the current retirement system, some mustangs
_who had made it to SNCO, but
_were passed over twice, and
_would be separated less than two years from retirement,
could choose to revert to their prior enlisted rank; if and only if the MOS was short SNCO's.

I never heard of a punitive demotion in the modern Marine Corps for an officer, unless the act would be a felony in the civilian world. In that case the officer is busted to private and forfeiture of all pay and allowances for the term of incarceration.

If the act by the officer would not be a felony in the civilian world:
_Prior to the UCMJ. It was not uncommon for an officer to lose lineal standing, effectively changing his date of rank.
_Since the UCMJ, an non-felony act act meriting serious discipline results in forfeiture of pay and allowances, a fine, and restriction to on-base bachelor officer quarters.
_More often than not, if an officer does some something that would merit demotion for an enlisted nan, he would be separated.

Can be done? Yes. Make sense? No, unless we're in an all out war. If you do something bad enough to warrant that type of butt waxing, you'll never get promoted and are just a loose wrench in the gears. I've been around several who resigned in lieu of something worse happening. It can play out differently if we felt someone at year 18 shouldn't lose the retirement as well. Those got a "creative assignment" to serve their penance. So what gets you hammered? Oh, night putting a female contractor on work you administer. Putting a uniform on and protest the Representatives office on the 6 o'clock news. Lots of punitive actions over the Tailhook Scandal. Those scandals are so bad, some got over punished, some got punished correctly, etc. Some got punished for not having anything to do with it. Hell, my about as far away from an aviator as I can get promotion was held up because I was required to prove the negative about being at least 200 miles outside of Reno. "You had two days of leave during this timeframe. Prove you didn't get on a plane, fly to Reno, and assault a service woman. Can't? Sign and swear under perjury this affidavit. Now prove you're not lying. Can't? Here's another affidavit." Wound up getting wound around that axle three times.