House hacking gives you the ability to learn firsthand what it's like to be a landlord, offset your mortgage using the other unit and to reap some tax benefits.

You can, but you will only have 1/3 of the total cost segregation benefits, and only for one year.Read this: https://www.biggerpockets.com/forums/51/topics/1122635-the-s... 

This group does allow syndicators to present deals, so your father would get deal flow, but he would also benefit from a wide group of other passive investors discussing the deal and sharing their thoughts.

I'd discuss how it would benefit them to lock in the amount with a lease. 

I agree with Jack that assets are great to have, but I could also see the benefits of having that kind of capital to really invest in a solid STR down there.

In this scenario, you will benefit more by holding off to qualify for a standard primary residence loan and house hacking. 

My family also has benefited from really high appreciation in the last 10 years due to a lot of northern Europeans, specially Germans, British and Swedish people who have a higher incomes and net worths than locals, buying a lot of the real estate in the nice locations. 

Appreciation and cash flow make up part of the calculation of the Internal Rate of Return (also including depreciation benefit and amortization of the loan).  

The benefit to the flipper is that they have extra working capital (and we do confirm they have the funds).

Unless you have photo proof of the condition of the property when you purchased it, they can say it was already there and the judge will give them the benefit of doubt.