you dont need to remove them to test you can disconnect the wire from the top then flash the top of the thread with a wire from the positive on the battery if you get a spark the plugs ok if you get nothing the plugs knackered
theres no inbetweens there either good or knackered
Not sure that the Vauxhall ones fit, there are two types fitted on the Mercruiser one is for Sterndrive applications one for inboard versions. Not sure what the difference is. They are not cheap! Retail price is £99.33 plus vat each we have 4 in stock at the min. You get 10% off that price.
Looking at your pictures, I can't see any obvious Glowplugs, unless they are the items you can just see beneath the injectors that appear to be connected by a tube? First glance it looks like a spill rail, but could be a bridging cable connecting glowplugs?. Normally you would have a heavy feed wire providing the current for them, can't see that either. Some diesels have a heater element in the inlet tract, but I can't see that either.
I am not familiar with this engine so am unable to offer any other advice.
A dc clamp meter can measure the total current on the glow plug circuit telling you how many are working... With out disconnecting things ..If you know what each one should draw... And you can find them