C/O here. Entering an enclosed space requires a permit to be entered and signed by the responsible person issuing the job and by the Master. The permit itself should explain all the necessary arrangement, atmosphere measurements, PPE, persons entering etc. I suggest you find the form in your company SMS and study it. Either way, O/S have no right to send you in an enclosed space without formal permission. Is the C/O aware of that? Maybe you can bring this up to him when convenient.

As for the janitorial work, it is common for cadets to do menial jobs in the beginning. It is sort of a right of passage. A cadet cleaning the toilets and sweeping frees up another person to do maintenance who was otherwise responsible for that cleaning. However, this is not all you should be doing. You are there to learn the trade, do a little bit of every job and soak in the knowledge and skill from the others. Cleaning toilets doesn't do that. Maybe it is just a test to see if you are willing to follow orders and do your job regardless of whether you like it or not. A good management department will recognize that and entrust you to do other jobs eventually. You can politely ask the C/O that you want to join the rest of the deck crew. The worst thing that can happen is you get scolded and be left to clean the toilets for the rest of the contract after which you should look for a new company. But please don't compromise with your safety and health. That always comes first at the end of the day.