You may be right about the cap, I've heard some have red caps, but I've yet to see any myself. The auto has a "lock" on it so you can't get the cap off unless you break it. If the red cap just slides off and there's a dipstick tube then that's the one.
Yep, Mercs can still be drained from the top, infact they are designed to be drained from the top.
That's a good idea to change your oil and filters more often in those conditions. It's worth it considering the value of the car
