The clutch can only slip in gear because that's when it's working. It can't slip out of gear.---;)

The rattling could be the springs in the dual mass flywheel.

However a slipping clutch means the driven plate is worn or the cover plate springs are very weak.

Was the cover plate renewed 15K ago ??

Engine mounts are still almost like new suprisingly after so much time, checked them immediately after feeling the vibrations...

Pressure plate was also renewed, yes, along with disk, master and slave cylinder (metal ones). But really now how could a new clutch disk wear out literally so fast to the point where it slips already? So even if it's in gear it still could be either the flywheel or the clutch disk you're suggesting? But since we already know that the flywheel is causing the vibrations, wouldn't it be reasonable thinking that the flywheel is also causing the slippage and that the clutch disk can still be fine as it should with no premature wear? I am just asking now if this could be possible in my case, not if it really is so, of course no one can say for sure, only estimating :shrug:

a small update on the matter, I started the car this morning to see how it acts while idling and no matter if it's in gear or not, the whole car is vibrating/shaking, and I don't think other transmission parts could be responsible for that. Or could they?

Is this happening when the car is stationary?? with the engine running

If the engine is vibrating and shaking could be a faulty injector or valves causing a misfire.:shrug:

Yes, when it is stationary and idling, but then again... if we take the fuel injectors hypothesis, how could those create the clutch slippage when driving? And it would be too much of a coincidence if the clutch and one or more injectors would choose to pack up at the same time..

When you drive the car does it drive normally or is it sluggish and feel if it is "missing"?

It feels "normally" regarding the way you're asking me, trust me I know what a misfire sounds/feels like, experienced those about 2 years ago, not the case here :shrug:

Sorry then, I have no more ideas at the moment.:shrug:

Well, my bad lol, it is actually a misfire, checked with toaf and it confirmed, it shows cylinder nr 5, as far as I know that's the middle one, closest to the front bumper to say so right? And also, next thing to do would be to switch coil plugs and see if the misfire goes to another cylinder right?

Actually nr 5 is the right one at the front looking from the front of the car... anyways, swapped coil pack from nr 3 to nr 5, misfire remained on 5, then swapped the spark plugs from 3 to 5, same thing, even swapped the leads from 3 to 5 and the misfire is still on nr 5. So what are the options now? Bad injector or no compression on that cylinder or what else?