I really don't know what that bolt in the floor might be. It could be something used once to bolt down something, it doesn't sound like it is plumbing related, but it's hard to say for certain from the description. If it was mine and I was doing it, here's what I'd do. It may seem like more work that just patching what you have, but you very well just run into one problem after another and spend more time and money trying to fix the old than just tearing out and replacing what need replacing.
I'd cut out the floor around that bolt and find out what it is and what is under there. Since the floor has a depression, the subfloor is likely rotten and needs replacing anyway, so cut it out, deal with what's under it and go from there. You make too much to do about cutting into the drywall. Drywall patching is one of the simpliest things a home owner needs to be able to do. It's likely or at least possible that your standpipe and drain are just 1-1/2", but new washers need a 2" drain, so while everything is opened up, see that your drain is 2" all the way to where is joins a larger drainline and that it is properly trapped and vented. I'd put in a new outlet box with a single lever shutoff that cuts both hot and cold.