New seals don't need bedding in. That's a cop out for sure!

The problem that will crop up over time is the oil seal wearing a groove in the shaft, and of course the oil seal itself wearing. In either case you'll get oil dripping from the telltale hole.

As for the mechanical seal. These seem to last a very long time, but when they go bad they cannot be salvaged. In the case of a bad mechanical seal you'll see coolant dripping from the hole.

Replacing the just the oil seal is a non starter as you must remove the mechanical seal to remove the oil seal. Bit of a pain, but that's the way it is. And removing the mechanical seal will surely destroy it, so that'll need replacing too.

Clean up the seal housings prior to refitting the new parts. They're generaly pretty dirty and you don't want to mess the new seals up when you pop them in.

The oil seal is pretty straightforward to fit. Can almost press it in my hand. Just make sure that it's nice an flat once installed. Note the correct orientation of the old seal before you press it out so that you get the new seal in correctly.

The mechanical seal needs a bit of precision to press it in. It's possibly to distort the shape of the steel housing if not pressed in squarely, so proceed with a little caution. I only ever do this on the hydraulic press, so that I know for sure that it's perfect.

before fitting the shaft, make sure that there's no roughness to the shaft where it'll meet the oil seal. Don't want to destroy the new seal before you've even got to the end of the road.

Once the impeller is all tightened up there should be a lot of drag when turning the shaft. This is actually a good way of deducing the condition of the mechanical seal. If it's easy to turn then it's probably on it's way out. Very similar to the center crank bearing on an MC21/28. If you can spin it, it's dead!

Hope that's helpful.