1- Can they ask questions in business maths where the compounding period and repayment periods are different?
2- How do assessors mark "describe the relationship between (box plots, two frequency tables etc.) questions, are they looking for key words, appropriate figures quoted? (and any tips on how to write these to ensure full marks?)
thanks 
1. Do you mean that it may compound monthly, but they want repayments over weekly? Pretty sure they can't ask that because the calculators cannot work that out, so I would say no.
2.They are mainly looking for, as you said, key words like 'symmetrical' or 'outlier at 6' or 'bimodal' or something along those lines. Also where appropriate they may be looking for figures to back your statements such as 'bimodal at 2 and 6' or for a two way frequency table they may want appropriate percentages. It really comes down to what the question is, and your own personal judgement, but if in doubt just try and get as much relevant information into a concise response making sure you are never being to vague and allowing the assessors to question or doubt your response.