The only reliable way would be reconstruct the floor of the mouth in some manner so as to separate the oral cavity and the neck. The 'salivary leak' that is taking place under the flap is one of the most problematic areas of NEck Dissection done in continuity with the oral cavity (tumor). The problem in this case being the potential risk fo 'contamination' of the neck with the oral fluids and bacteria with potentially fatal complications like even Carotid Blowout or other vascular erosions or flap failure / necrosis. But reconstruction of floor of the mouth is one of the most difficult aspects of post ablative reconstructive surgeries due to lack of local tissues available. Free Flap reconstruction might be the most viable option at the moment, in my opinion.

Go for a tight layer by layer closure.... this would prevent saliva leak. Even if there is salivary leak post operatively, put a compression bandage. It will be resolved in few days.

Sialocoele following radical neck dissection is most probably from the tail of parotid which gets involved when level II nodes are cleared. Hyoscine transdermal patches or BOTOX injection to parotid bed might be a viable non surgical alternative.