Cap job - grounding ?

[Dave Curtis, dB AudioTech]

And that first picture with the soldered ground wire is additionally incorrect as the grounded wire needs to be the longest one of the three power cord wires so when or if you rip the power cord out of the chassis/strain relief, the ground wire is still attached to the chassis, keeping it Earth grounded.
That way if the hot black lead touches the chassis on on the way out... it blows your home/shop/club, mains circuit breaker or fuse, ....
saving the possibility of you waking up in a puddle of pee.

True; that's a good way to do it, but it's not required (by the NEC, anyway). A short ground wire doesn't violate the NEC as the solder-only ground does.

[Dave Curtis, dB AudioTech]

Two questions:
1) What's the reason for this (I mean, bolt plus solder)?

The solder is extra insurance. It's not required, as is a secure, mechanical connection (nut, bolt, lockwasher, etc). The NEC says ground connections cannot rely only on solder for a secure connection. I realize that you're in Italy, but the standards there are likely similar.

2) I have used in one or two occasions the mains ground lug/bolt on the PT also for grounding a filter cap. Is this bad?

The equipment (AC cord) ground connection should be separate from any other ground connection. A filter cap's ground connection should be physically close to the power transformer's secondary grounding point; ie, the center tap or the negative node of the rectifier...