As far as I know, the 3510HST is the same as my 2610HST in this regard. As sea2summit points out, the owners manual on page 7-21 lists 5, tractor only grease points to be done at 50 hour intervals. I'm inferring that the PTO lever you reference is actually the three point lifting lever (position control lever)?
1-2 Fore and aft on the front axle pivot- always discussed here as to how difficult it is to get grease to be accepted. I only have 100 hours, but after three attempts lifting the front tires off the ground and/or pivoting the axle as suggested hasn't allowed me to inject any appreciable amount of grease in these.
3. Three point adjustable side mechanism
4. The three point lift lever ( 'position control shaft'). It was a pain for me to find, and couldn't be reached with backhoe frame in place IIRC. I'm not going to sweat doing this one very often, but I accessed it as you've described.
5 The 'Not Clutch Pedal' grease nipple is perfectly hidden from viewing from the side. See photo
For what it's worth, toukow