Besides it looks like espostos siding which nailing into isn't easy or recommended. 

There's been some movement in the slab, and I noticed the nails on the braces connecting the floor joists to the central beam have loosened a bit.Thanks in advance!

Nail on the head @Jay Hinrichs.  

Most banks here in Canada have a cap on the total  # of properties you can own.Scotia - 10Desjardin - 16 doorsCIBC - 10TD - 14It does not matter the total $ amount of mortgages - it's the number of doors. 

Next I'd nail down the subfloor panels and lay a rubbery cork like material over subfloor.

Do I want to buy the house for more that $450,000 (if I'm given the choice) so that when I sell it and there's a gain I won't get nailed in taxes? 

It's advised to inform these tenants that they are living in non-conforming units, so if anything happens, they can't nail you on fraud either.  

So the cap on Title III is not that bad, especially if you think of the hundreds of flippers on BP that may want to take advantage of the new rules to raise capital for their first or next project. 

The contractor knows that investors will fight tooth and nail to not pay for a change order (and he investor may not make money if they have to pay the change order).

@Brent Coombs nailed it.