If the invoice is real and the plumbing company replaced all the piping in the house, then I imagine all that work is under warranty, correct? 

It is funny because on BP we try to describe how the Bay Area is pretty different compared to other markets (I can only imagine dealing with a M2M in sub zero weather and snow during the harsh winters in Wisconsin).The issue you can run into for the M2M is that less tenants might be interested (more might be as well, just depends).

Imagine our surprise when a former tenant who owes about $1200 in damages beyond what her security deposit covered contacted us asking about setting up a payment plan to repay!

Imagine trying to evict for non-payment and they claim they sent it and you just never deposited it.

A month ago I could not imagine there such web site as BiggerPockets would exit.

I would have to imagine what someone would think driving by and seeing that sign there.

As for your other questions, I know that the large majority of AirBnB properties include a mandatory cleaning fee, and I would imagine that you would treat pets as you would in a SFH rental (additional deposit, etc.).  

Despite all that, I couldn't wait any longer to chase my dream of flipping houses and building a larger real estate portfolio and hope that I can make the transition over the next year making this my fulltime engagement.Redid my first home and love dreaming, buying, and executing what I can imagine in my head. 

But from my experiences as a home owner and knowing some prices of properties around Bremgarten and Zurich, I cannot imagine that the ROIs are comparable to the US.

People have come up with a lot of work-arounds like "leasing for a month and cancelling 3 weeks of it", but if brought up in court well you can imagine how that would look.