The estimate selling market for the house is about 410k.

I'm for now just going off the conservative estimate of a very BROAD ballpark from a friend who works in insurance.  

The reason my loan delayed (generally it takes 7 days in our case it took 18 days) is due to Covid-19/refinance issues for lender and the reason my loan denied is due to change in lending terms for Jumbo Loan they requested 20% down and 15 months of reserve (Lender Estimate is for 15% down and 6 months reserve).

The evaluation tools on this site were instrumental in keeping in line with the estimated cost.  

And the 850-950 euro estimation is based on also long-term renting but in a bigger market?

Without a ton of info I think the landmines are dependent on how accurate your valuations are and how accurately you can estimate the cost of building the homes. 

When I account for taxes, vacancy, and management expenses on top of maintenance and upkeep, I'm wondering how many landlords are able to actually profit well from their properties using reasonable financial estimates.

The purchase for that property was estimated to be $80,000 by the person selling me the building but turned out the best I could get was $150,000 with the actual seller. 

Press the agent on the big ticket repair and see if any estimates for the repair have been done by the receiver. 

It's all an estimate but I think that some of the figures you have are high.