You are pretty much spot on with your presumptions.
But anyway, here's a few things to consider:
• CO2 has a far lower melting point than water, meaning it will not solidify under normal domestic-freezer conditions, and so will remain as a gas.
• The sugary syrup, containing large amounts of glucose, also has a melting-point lower than water, meaning it too will remain as a liquid between ice crystals.
• Molecules are very, very small.
• Ice crystals have a lot of 'empty space' between them despite otherwise appearing entirely compact, This is due to water's natural molecular structure, in addition to the presence of tiny air-pockets formed during the freezing process (explaining why H2O expands when a solid). Hence, there is still liquid in the bottle.
• Provided the bottle was sealed when frozen, the pressure within - though reduced due to the cold temperatures - would still superseded that of the outside world (the reason carbonated drinks schhhhhhhhhhhh when opened, as the pressure of the vessel equalises to the atmospheric normal).
So we now have a bottle containing frozen water and a liquid sugar syrup, in the presence of gaseous carbon dioxide - with all components loosely locked into one big crystal structure under pressure.
You open the bottle... Immediately, the pressure difference initiates the cool schhhhhhhhh noise, rapidly equalising the vessel to the standard atmosphere outside and releasing its CO2 in the process.
To escape, most of the CO2 must first pass through the frozen water - which it can do on account of being so small and the ice crystals having numerous little spaces between themselves. The migration of hundreds of thousands of CO2 molecules through the ice results in the passages, holes, 'tunnels', forming.
The CO2 is not alone in the journey, mind, as locked within the frozen water with it is the liquid Pepsi syrup, that - caught up in the movement of CO2 - also utilises the tunnels to rise up the bottle neck, pushed up and out of the ice by the force of the escaping gas behind it.
This fizzing process concludes once the pressure of the vessel equalises with the outside environment, halting the movement of CO2 and with it: the syrup, which now exists at the bottle's opening, and all over your hands...
Delicious. 815