Fear of harrasment/ threatening behaviour claim?

If it comes to legal blows, remember: EVIDENCE, EVIDENCE, EVIDENCE.

No judge will want to listen to 'he said she said'.

Photos, bodycam (assuming that's not a variant of harassment?), obviously a carefully kept and FULL file of letters, emails etc.? Legal 'Disclosure' involves producing ALL the docs, even those that may be against you. By which time you will be talking to m'learned friends?
Questions:
1. you mentioned guarantors shouting, are they worth powder and shot if you go to court and win and the tenants do a runner?
2. have you tried simply ignoring them? Is this just a silly game that they played on previous landlords - did you take up references?

I can't see any grounds for a rent repayment order. However I can see how you might approach things differently to avoid some of these tensions. I too am a student landlord and have licensed student HMOs. I have noticed over time that good beginnings are critical to assisting tenants bond with their new home and settle and also reassure parents. That would include making sure the property is clean and in good decorative order and all appliances, furniture are working, stain free and free from damage prior to move in. I paint all walls white and patch paint if whole rooms don't need redoing. I would either repaint or spray with damp seal any small patches of damp. I use H20 to remove mould cause by lifestyle issues. If there are serious damp issues then yes I would get damp work done but this would be prior to tenants moving in. Particularly during the first weeks I jump on any issues that are reported to give tenants and parents reassurance that I am responsive. I employ an agent and try my hardest not to get directly involved with any communications with tenants. If I need to visit I always give 24 hours notice. I try to repair/replace appliances within 24 hours. 9 days without a fridge is unacceptable in my view. I usually find by mid October things settle down as students become distracted by University life.

It does indeed sound as though they have heard of RROs, like the sound of it and are hoping to move the situation towards that, but by the sound of it in reality it seems unlikely that the situation warrants it.
I would say just remain polite, businesslike and responsive without giving any impression that you are worried.
Hopefully in a few weeks time they will be working hard revising for the May exams then Summer will be round the corner and they will be moving out.

There should be a heat/smoke alarm in each bedroom plus others in the communal areas and hallways which must be tested as per the requirements in the Fire Regulations - I presume your agent is doing this and keeping a record.

Did you not do a thorough clean before the new tenants moved in? Why on earth not. If it was dirty I would have been complaining lol.

For communal areas you have right of access for you and/or tradesmen for emergencies. I would give 24 hours notice for any other reason - which on some occasions you have not done.
I would give 48 hours notice if you or your agent/tradesmen need to enter an HMO bedroom, unless an emergency.
You did not do this and it could be seen as a breach of tenants right to quiet enjoyment of the property.

"There is in fact no chimney for decades" has the chimney been capped off and the fire breast vented? If not then you might get water ingress and damp.

The smoke/fire alarm needs to be tested weekly and your agent can state an inspection of the communal areas will also be carried out at this time.

Hope your agent took photos of this "‘What to do this week notice board’ which stated phoning the letting agent one day to complain and another was to send a passive aggressive email to the agent again."

How did you know "This tenant also moved the desk from her room and replaced it with the desk in the spare unused room without even asking." Was 48 hours notice given to do a room inspection? If not you are again breaching a tenant's right to quiet enjoyment of the property.

The students dont need to produce the evidential photos until it coming to moving out and not getting their deposits back. Doodling on the fridge, if it doesn't clean off, could be seen as wear and tear, or just make a small "reasonable" deduction from every students deposit when applying to the DPS (or whoever you have protected the deposits with).

"the chance of a harassment/threatening behaviour claim by them and thus a possible rent repayment order." Have you ever threatened the students? If you haven't given the correct notice, and not only is this legislated but should be a written clause in the tenancy agreements stating the amount of notice that should be given, you have opened yourself up to a possible claim should it ever proceed to a judicial hearing. Always abide by the clauses in the tenancy agreements and legislation even if you think you are being helpful as you can be seen to be denying your tenants the right to quiet enjoyment of the property.

Always have a week turnaround between academic lets to do repairs and maintenance and to thoroughly clean. Most student properties are in need of a thorough deep clean and maintenance at the end of the academic year!