Your post doesn't indicate how long the rear decklid will stay in the raised position before it self-lowers.
The front struts aren't hydraulic; their pneumatic, and as you know, are held in place by clips. They're easily replaced in a few minutes.
The rear decklid hydraulic cylinders are hydraulic and as such, have supply and return lines. Inspect the rod seals for any hydraulic fluid leaks. Verify the hydraulic fluid level is between the two indicator lines on the reservoir on top of the hydraulic pump manifold in the trunk. A low level is a good indication of a leak in the hydraulic system.
If you replace them, it's a good idea to replace their O rings as well. If the rear decklid is going up and down as it should, your hydraulic cylinders are probably fine. I have always been able to lift or push my rear decklid shut.
It's always possible you could have a leaking solenoid valve, which are located on the hydraulic pump manifold. These are replaced as an assembly (not cheap).
Since you mentioned this problem occurs when it's cold, the hydraulic pump has a thermister that prevents it from operating in too cold or hot temperatures. A warning message should also be displayed on the DIC if this occurs.
If in doubt, I'd recommend having the system checked out by your dealer.