Badali,
I preface this by saying that I'm not 100% sure about this, but...
The 3 three photos are indeed factory mounting points. The last photo, however, I'm pretty sure is a bodge job by a PO in years past. The shoulder mounting point at the B pillar is a strong point, as is the tunnel. It seems to me, however, that the floor mounting point is at an inherently weak point in the car.
I've put 3 point belts in my car, which hitherto fore had just lap belts (it was a US late 230). I followed MB's directions in their bulletin, (I was sent this by a fellow forum member, and I've sent these to many others). I also used the correct MB fitting kit. At no point was there any mention of a set up like yours.
To that end, I'd strongly recommend that you don't rely on the floor mounting point as it stands. It seems to me that the mounting point is on a floor pan - usually one of the first things to rust and be replaced. From the photo of the underside of the car it may be that there is more strength there given its close proximity to the training arm, but I'm pretty sure it isn't factory - but I can't say for certain as I just don't know enough. I am, though, certain about two things: First, I wouldn't rely on that point as a seat belt anchor, and second, it isn't the way MB say to do it now.
Sorry to be a bearer of bad news, but I think the car isn't as safe as it could be notwithstanding that you're replacing some pretty knackered old webbing.
JH