1. It would be best to start with what you remember of the conversation yourself. You can then put that to them and see how they react. Advise them to retain any recordings. You will not get these without court action.
3. Most banks got wise to using these after 2005 but I am not sure about this credit company. If they haven't gone through one of these with you your task is easier.
4. No, there should be no penalties attached to it. It is likely to reflect sums taken already at the point at which you cancel. They can be quite arbitrary in their calculations so the best thing is to ask how they get to that figure.
It would be useful to get a copy of your agreement to see how the interest has been charged and any penalty for early redemption. I am assuming yours is a CCA 1974 section 16 (6C) agreement if it is secured on property and it is therefore regulated by the FSA so the Unfair Relationship provisions in CCA 2006 (incorporated into section 140 (A) CCA 1974) won't apply. There are, however, various banking and other codes which may apply. You may also have an argument on Unfair Terms but the starting point is how you got into this in the first place. All PPI cases are at bottom evidential. Mis-sold PPI can taint a related agreement.
Have you thought abouut consulting a solicitor? If your PPI is over £5k (and I suspect it will be on a loan that size as it is usually 20% upwards) your claim could be cost-bearing. The Internet contains details of solicitors who do PPI and CCA claims and most will advise initially free of charge just to see if there is a case. Where there is a home at stake there is often more of a case than in other situations.