I think there may be a problem using more than 4 strands of paracord - it is quite thick. You might try a 4 strand sennit (there will be many YouTube videos on this) and for the 4th strand add a neutral colour (black or white maybe?). You could try a 6 strand sennit using 2 of each colour but this may be too thick unless you remove the core of the paracord first (sometimes called "gutting" it).
To make the lanyard make a long piece and in the middle slide a clip to hold the whistle, make a small hanging loop by whipping the sides together (and cover with a 3 part Turk's Head?) then add silver(?) connectors to the ends. Simple but effective.
Sweeney