Don't know what's going on with your throttle and choke but just wanted to mention something I've learned to help with your stalling issue until you can get it sorted out.
Instead of just engaging the PTO and leaving it engaged, pull it out for a second to just get things turning, push it back in and then pull it back out. This works for my somewhat tired Exmark running heavy G-6 blades with the spindle-driven vac unit on. Basically it splits the start-up load to 2 events rather than one, reducing the strain on the engine.