They had 3 young adults living with them ("day & nightcare" ) not ready to be independent.

You need to see out other Agents (preferably Independent Agents) for quotes.

What you can invest in depends on what the plan documents allow - the plan documents of one plan provider don't allow for investing in real estate (i.e. the brokerage) vs the plan documents of the other Solo 401k plan provider do allow for such investments. 

Financial independence in 5-10-20 years?

Real estate is a perfect vehicle to put together blocks for financial independence.

Can I just be an independent contractor as a 'care taker'?

I would always get a quote from an independent mortgage broker

My dream is to leverage this gift into financial independence through multiple income properties over the next 10-18 years before I retire. 

This implies that using the chimney vent to run wire is a no-no; each system needs to be independent of any other subsystem in the building.NEC 300.22 specifically covers wiring within spaces that handle exhaust or environmental air.

I am not sure how that works for EU citizens--I have been learning about the Portugal option from US citizens who have achieved FIRE (financial independence retire early).