The v6 cdi has a different set up to your 612 engine. Earlier cdi's have a fuel feed pump which relies on a vacuum being held once the vehicle is switched off.
Unless there is a noticeable fuel leak from a line, it's unlikely for o rings to be the cause. The piezo injectors which are used are also far less likely to return excess fuel and cause a rail pressure drop on start up.
The om642 engine used in the v6 cdi's has a lower compression ratio than earlier cdi 611,612,613,646,648 engines and does need pre heat to start efficiently.
Hope this helps