WTF-- temp drops below freezing(0c) and you cannot move the shift lever into any gear. You cannot move the shift lever into any gear engine off or on, clutch in or out. It will move sideways in the neutral "slot", you cannot force it into any gear, Once we get above freezing things are fine. I have been a tech in the distant past and I have never come across this problem--ever. Water in the shared brake/clutch reservoir doesn't make sense--you should still be able to get into a gear with the engine off.
Most probably nothing to do with the clutch or gearbox but the cables connecting the lever to the gearbox to push/pull the gearbox selector lever are freezing due to moisture inside the cables, they most likely need a good spray with easing oil and a grease up??
All you could do easily is from the engine end, get the engine hot or go under the car with a hair drier to warm the cables and squirt a bit of WD40 in at the engine end whilst working the lever and hope it travels along the cable??
If no good the gear lever need dropping out and do a proper job, or hope the sun comes out?
How about some bright spark having had the idea to spray WD-40 into the cables to improve the shifting action? I hate that stuff because it's hygroscopic so while it initially attracts moisture away from other things (like ignition circuits) it keeps attracting moisture until it's fully saturated - so then you've got a little water reservoir wrapped round everything that it's been sprayed on, just waiting to freeze up.
I claim expert knowledge, having run Alfasuds through British winters - the combination of salt sprayed on British roads, Italian electrics, long plug leads to the flat-four cylinder heads and long intake tracts from a single central carburettor made starting one of these a horrendous task that some people were dumb enough to use WD-40 to 'help', thereby building up a guaranteed problem for the future.
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