Not really sure about the ethics and all that but I'd stick to lifting up the grill.
If you go through the manhole you have to make sure you mark it. If you don't you might accidentally open the wrong one when returning, and that could be bad, especially if it opens up on a road or something. It also makes dealing with a burned out flashlight that much harder because now you got to find your mark in the dark. Then of course there's the very small possibility that someone reseals the cover with you in.
Also I'm not sure exactly how the law works but I wonder if you will get into more trouble for using tools for your trespasses. I expect it's a lot easier to prove pre-meditation if you got a tool with you.
Lifting up the grill is probably better. Even though it's exposed most people probably wont care what you do. If you're really worried about people snitching go early in the morning. Though tbh even if lots of people are around you probably would be good. Oddly enough people are less likely to report crimes when there's a higher concentration of people around. It's called the bystander effect.
You could even buy a really cheap construction vest from Canadian Tire so it looks like you are supposed to be there.
Have fun