Take the two hose off that go into the bulk head. Then get the hose pipe in, hold it in there and get some one to turn the tap on. Finished flushing when water runs out clear. Do it on both sides of the matrix pipes
I'd check your linkage and the sugar cube under the selector arm on top of your gear box first. If that ok your going to have to open the box up. Take the end cover off and see if reverse gear has fallen off
You can put it on close to your position of how your one is so it would run. But I wouldn't boost it till you have checked the timing with a timing light gun
I do think your thinking about it too much. Just let the breaths vent to atmosphere. They only really return back to the engine to reduce emissions and smell