this is one of those high liability questions! I will say that I loosened the oil cooler attachment first this year, and lots of water came out. I then loosened the same petcock and no more water came out. So, in my opinion you're probably okay leaving it in there. But, I would still get that petcock out just to get it out.
I had a similar problem with that petcock (it's in another thread in the last couple weeks) and not being able to remove it. If you can't get that brass bolt out of the petcocki, you should be able to remove the whole assembly as it's threaded right into the block. You have to remove a couple things to get in there, but on mine you can barely attach a socket (plus 24" breaker bar, if necessary) to the end of the petcock assembly to break it loose. I will admit that it was stressful. You don't won't to break that assembly at the block!
BTW- don't forget the direct drive engines are 180 from the V-drives. I'm assuming you have a direct drive....