I was going to ask questions in the comment section such as "What country are you working in? Are the weekend hours in addition to the 40 hours a week you spend developing code? Are you compensated for those hours? If the support hours take place during regular business hours are you also expected to code during that same time? On the weekend are you at home or in the office?" but those focused on how you might want to preserve your job.
Therefore I decided to focus on your question
"Given the current state of the market, I'm considering moving on, and
would appreciate any advice on how best to portray this short stay"
I would start to apply to other positions. It has only been a few months since you rejected the other interviews so it is possible that similar positions remain open with the other companies. I would not quit until a new job is found.
I would be upfront and list the job on the resume. Skipping it would could cause problems later on. They might want to contact your previous supervisor. Normally they avoid talking to your current supervisor to make sure they don't get you into trouble. It is true that many times a company will only confirm dates of employment and job title, but if they do get in touch with your previous manager he could say that you left to go with company X. And Company X isn't on your resume.
It could also be found if you have to fill out a form for a background investigation. They might want to talk to co-workers at your old job, this makes it even more likely that you will be discovered. Not including it on the background investigation form could get you dropped immediately. You can't say that was a temp position 10 years ago, it was the place you went to the day you filled out the paperwork.
One short duration position is not a red flag at all. It can be a sign that the economy, or sequestration forced you to leave a position you liked, for one that promised a decent paycheck without any unpaid weeks or months. Now that you are on your feet, and can see reality in a calmer environment, you need to find a job you like.
If they ask why you are leaving, tell them that the job was advertised as a developer position, but it is clearly evolving to a help desk position, and that I am not comfortable in that role.
It true that some employers will see the short duration position as a problem, and never call your in for an interview; but some will see the two months or more of unemployment and never call you in for an interview. I have no idea which pool of employers is better to avoid: the minority that you eliminated because of the truth, or the minority you eliminated because of the omission.