According to the Fédération Aéronautique Internationale (FAI), space begins at 100 Kilometers (62 miles) but what would be the lowest possible orbit?

Soviet Zenit spy satellites launched from Plesetsk had a perigee of 150 km and were recovered after 14 days...
US Keyhole photo-reconnaisance satellites equiped with Agena's restartable engine could maintain perigees as low as 135 km for 50 days!

Following circular orbit theory, a satellite must be travelling about 7.7 km/sec, this equals an orbital height of about 200 Kilometers.
Once a satellite is down to about 110 kilometers circular orbit it will usually impact into dense atmosphere within the next revolution.
Break-up occurs at about 80 kilometers...

What was the lowest "usable" orbit ?

How many orbits is usable?
Also, this will depend on e.g. the solar cycle, since this will cause an expansion/contraction of the atmosphere.

There is no lowest "usable" orbit. Lower and lower orbits rapidly encounter more and more air drag and are more unstable. Depends how much propellant you have to fight the drag. Atmospheric density at those altitudes varies quite a bit with solar activity so your mileage may vary.

Google search came up with:
Carol Laymance: Space begins at the lowest perigee of an orbiting satellite, about 93 miles beyond Earth’s surface.

That's about 148 kilometers... the Russian Zenit spy sats were the record holders I believe
US KeyHole cheated by augmenting the orbit with a restartable engine...