Hi
Yes, I would suggest building your model and audit trail side by side. For example, do the data checks, document them, add in your parameters, document them etc.
I think another user gave some good advice on another thread on this, but, yes, I would keep going even if what you have doesn't look right. it is better to have worked through everything and be able to document something, even if it isn't quite right. You can always use placeholders (eg a hardcoded value) if you're really struggling with something to allow you to move on.
I wouldn't hide that it doesn't look right from the examiners as this demonstrates that you've done some reasonableness checks. Remember that in Paper 1 you don't need to interpret/discuss the results (other than checks) so it shouldn't be too big an issue. Hopefully in Paper 2 you'll be given a start so it won't go too far wrong!
I'm not sure if there are half marks but you should still earn follow-on marks for getting the next step(s) correct.
Sarah