Not meant to be nasty, but how do you come to the assumption that the construction went smoothly? How can you be sure that almost everything doesn’t have some kind of defect? Your two points really shouldn’t occur. On the one hand, the electrician would have created a proper test report if the error had been noticed. The same with the sealing; the installer had leftover parts with the attitude of “I don’t care.” I would now be much more concerned that much more is actually wrong here.
Certainly no single builder knows this for sure. We definitely had very good daily supervision on our part with relatively decent applied technical knowledge. Of course, it is certainly more difficult in the electrical area and you cannot monitor every step. The question is: who can? Our neighbors have a construction supervisor and felt like they had a new problem almost every week, which, however, could only be discovered afterward—that is, nothing was noticed during the actual work...
Without proper execution of the respective trades, damages cannot be avoided either—which means: you also have to “trust” every trade or at least hope that everything is done properly—especially works that are not that easy to inspect.
A loose wire (neutral conductor) surely wouldn’t have been noticed by anyone, or rather the electrician MUST necessarily check that. Apparently, someone was unwilling or took it for granted. These cases happen maybe 1 in 100,000 times—but unfortunately it happened to us.
The same story with the water damage. The line to the garage was kind of “kinked,” so they just took the last multi-branch line that was actually meant for gas; the sealing didn’t fit, and then the craftsman probably thought: “Oh well, it’s all sealed anyway, I’ll just leave it out,” instead of reporting: “Hey boss, bring proper sealing, I can’t connect it like this.”
Of course, there can still be problems everywhere that go "undiscovered"... But let’s be honest, can you really go crazy about it now? We were just unlucky in these two cases. Nothing helps... I don’t feel safe with this “electricity issue” right now and absolutely want an opinion from an independent expert—I will insist on this through my general contractor. At least then this issue is settled, or I’ll know what exactly needs to be done.