We bought a house from a plan. We met with the architect and spoke to a few people who'd worked with him.
He used the same sub-contractors on every build, and I think this really helped the reliability.
We visited the property often whilst under construction, and spoke with a lot of the guys working, explaining on occasion some background on changes we'd suggested.
We had one fairly substantial error caught before it became a problem, and were able to feed this back through the sub-contractor to the architect / PM without putting anyones nose out of joint.
I'm convinced that getting involved helped, we were very careful not to be annoying about it, and actually I think the guys we met were pleased to know that the work was appreciated by those that were going to inherit the completed property.
Only one issue : zoning rules... it was very quickly suggested by one of the outside builders that if we changed the plan with X that it would provide an additional parking space outside the house, and look better.
As the architect / PM was there we said OK.
Turned out that it was a blunder to do so. This changed the way the house was seen from the road, and the front door wasn't masked any more.
I forget why that was important, but it broke a rule and we had to pay to revert this - after the final bau kontrol. 6 000CHF in the end only for that work to the builder and the documentation updates for the gemeinde.
That was the only thing over budget in the end.
I learned a lot about construction, and made some really good contacts during this period. This is probably the key lessons we picked up - check often, speak to the teams. Knowing a lot of these guys pays off, as knowing to whom you can get advice, and realistic quotes is great. There's always some work you're going to want to do.
Any fundamental change to the external plan, no matter how small, be sure to check it doesn't affect any zoning assumptions. Ask at least.
Hope it works out for the OP. I would add my voice to those suggesting you get some help. Could be costly, but wow, the peace of mind and avoidance of serious concerns avoided would have to be worth it....
Get a PM maybe? Not sure what to suggest, maybe others can. I genuinely wish you the best, I can only imagine the stress that you're under with it all.
Cheers.
Note also that it's legal in CH to cover up to 10% of agreed price due to unforeseen costs ( labor mainly ) - on 1mCHF home it could be 100k at the end of the day - and no building company is going to walk away from extra10%.
Cheers.