Well as I'm known to be talking rubbish maybe you had better measure my words carefully, but I doubt enough diesel could get past the injector spray valve and into the leak-off system to do much harm, and if it did it's got nowhere to go except back into the low pressure side of the fuel system, it still has no way of getting into your engine crankcase.
If an injector had been dripping, and that only means not snapping cleanly shut but allowing a slow wet close-off affecting fine atomisation of the fuel, you would have been well aware of it through prominent combustion noise on the offending cylinder, as if a bearing was knocking, and with poor starting through inadequate atomisation, and loss of power.
Regarding the leaky plug you refer to, I think you mean the one injector leak-off pipe at the far end that is blanked off with a rubber boot, well I have found that the leak-off system isn't very long lasting on the TD10 I have, and I replaced all my leak-off pipes with black 4mm nylon pipe and fitted a new boot when the originals were showing signs of decomposition with diesel oil.
Run your engine without these leak-off pipes in place and you will get an idea of how worn your injectors really are, new or serviced injectors will leak very little, high mileage ones will leak a bit more, but don't imagine any of that can get into your oil sump because there is no route for it to pass, it has to go back into the fuel system.
Also bear in mind that diesel oil is also a lubricant and most small injector pumps are lubricated entirely by it, your Bosch pump included. So any getting into the engine is not likely to cause a sieze-up although slightly heavier bore and piston wear may result over time. But you would know about the problem long before, through white or blue smoke that diesel produces when inadequately burned.
Your caravan is obviously quite old, but old caravans rarely do big mileages because they spend most of the year parked up, and when on the move people want to spend time sightseeing so high mileages such as one gets in some trucks for example, are unlikely.
I drove a Volvo F10 tractor unit with a 10-litre diesel engine pulling a concrete mixer, for 10 years before my retirement, covering well over 600,000 kms and never had to have an injector serviced nor the injector pump in fact I had no engine trouble of any kind at all. Before I took the unit over it had spent 15 years hauling refrigerators at 40 tons between Valencia and Northern Europe and must have done many kms more than those I put on it.
So look elsewhere, your injectors are not likely to be faulty. If in doubt, ask the diesel shop to test them. If you hand them over in your hand the cost should be trifling as they have a hand-operated pressure pump that will give you an answer in a few minutes. Try to get a compression test done which will show up weak cylinders but with diesel engines low compression becomes self-evident especially at cold starting, it either doesn't start or starts with cylinders missing. If it starts up quickly with the injector pump cold-start knob in the advanced position then pump trouble can be discounted, and in any case the only connexion between pump and sump is via the high pressure system and is very tenuous, past the pistons.
But bear in mind I'm talking rubbish with only 50+ years experience of buying running driving and repairing diesel engines.