Hi, I have a Grohe Euromix 33619 shower faucet, and last night, the lever (part 46 029) broke and is hanging loose in the handle assembly. Two questions:

1) How can I access the inside of the handle? I've read some other posts on this site about this faucet, but all the posters have already removed the handle. Do I pry something off, such as the clear plastic cap on the front?

2) How can I shut the valve off temporarily while I order the replacement part so that I can turn the water back on in the rest of my bathroom?

Thanks a lot for any help.

such as the clear plastic cap on the front?
Yes.
Removing the cap should give you access to the circlip which attaches the handle and its supports to the valve stem. Handle could be broken, the circlip could be out of place or the end of the valve stem could be damaged.
After removing the circlip, the handle and it supports, you should be able to push the valve stem in to stop the water flow.

As said above: you start with popping off the cap. Here is exploded view so you know what to expect