Presumably. But for one LED or maybe two you probably wouldn’t need the transistor, the LM324 can source 20 mA min, 40 mA typical.
None I can think of. It’s often good practice to have a 100k pulldown resistor on module outputs anyway. (For one thing, if a module is unpowered, then its op amp outputs are probably floating and can cause trouble for a powered module they’re plugged into.) Though I rather rarely see it done. And a pulldown after a series diode is definitely called for in many cases. I think this is one. You might want to add a note suggesting it to Annotated Simple DIY Envelope Generator.