Hi,
I and many others use a "skrawker" such as this on plastic...
https://www.cottage-craft.co.uk/product_info.php?products_id=217I wouldn't be without mine from the legend that is Olfa.
...Youre right to run a pencil line down the moulding lines. If I were you I'd get it close to the line using cutters, scissors etc, then I'd "shave" down to the line rather than sand. "shaving" creates curly swarf -no dust until you need to create it when it come closer.
...I'm not familiar with this model so sorry for not undersanding your pictures of detailed parts but I build allot of models from lots of vac-formings. I'd be a bit cautious -sometimes these mouldings are supposed to mate with a corresponding lips on the other part so there's a secure layer for bonding. Don't cut too much of either away each-time before offering up and checking. In any case, adding some polytyrene strip in the join is always recommended.
Rich