Ficus benjamina have very problematic surface roots. They are nice trees but really not a good choice for a south florida home lawn due to problems with slab, plumbing and pool damage. As if that wasn't enough they are prone to pest/disease.
If I were you, I'd taken them out and replace them with an awesome native tree like Sugarberry.

Most likely we will take them out in about two years.

Right now they give us afternoon shade and some privacy.

We are planting other trees/etc between them. When the new stuff gros we will probably remove the Ficus trees.

I still need to know if cutting the surface roots will make the tree fall over in a wind... or kill it before I'm ready to take it down.

The short answer is "probably not", to either making them fall over or die off. Hurricanes coming directly overhead negate any guarantees, however :-).

Their root system is so massive, as I recall, that cutting it will probably have little effect. However, cutting roots is just like cutting branches - it encourages growth from the cut ends. I would make any planting holes a LOT bigger/wider that you might otherwise, to try to limit root competition from the ficus. Unfortunately, watering the new trees as they will require will also encourage the ficus roots to grow into the holes from the outside. You might want to push a shovel into the soil around the outside edges of the planting holes every few months to prune off the ficus roots and keep them out of the planting holes. The roots of the new trees shouldn't grow out to fill the hole so quickly that you would be also cutting them.

And, of course, I assume you will mulch the new trees, no mulch in the 2-4" next to the trunk, out as fas as possible, at least to the drip line, and no deeper than 4".

That is a good answer from Dibbit and it sounds like you got a good plan going. And hopefully planting something other than palms.