Re: to cut or not to cut. that is the question.
Cut the cleat, clear it out. See what's beneath it, i.e. any backing plate.
You may very well be able to install the new one by using LONG bolts, so they protrude down to an accessible area, and then using a basin wrench to hold the new nut on until you can get it threaded up.
If you've never seen a basin wrench, it is a "WTF did that break off?" kind of tool that plumbers use to reach up into the narrow space behind a sink, in order to tighten up the inaccessible nuts on the bottom of faucet fixtures. One size fits all, they're spring loaded to tighten down, and they'll easily go a foot or more "up" into an inaccessible pocket.
If you want to install coaming pockets, they're very useful. But if you just need access...I think a basin wrench, under ten bucks at the hardware store, is all you'll need. Or, long bolts, so you can get the nut started by hand, and then just have to screw it up several inches.