if you deselect A/T on approach are manual thrust changes still available?
Yes, If the AT is deselected (put into ARM mode) you have to use the thrust levers just like in total manual mode. AT will automatically activate in a few cases though, like approaching a level off altitude or getting close to bug speed with a speed trend for even less speed. That can be a bit, well, non-intuitive during approach in gusty conditions as it will activate well above Vref given a sufficient large trend, but in all other conditions it adds low speed protection which is simply an added safety net.
As said above ARM is simply a manual thrust mode, so keep your hands on those thrust levers, which means you can immediately counter any non-wanted thrust input should AT activate at the wrong moment.