Well that's the problem isn't though.

You assumed he was like any other old blood during the day.

When in fact he isn't. in Daylight he loses the boost, including his 'base' vampric speed and power in direct sunlight, as part of the backbite of his attunement to darkness. Making him just a very long-lived human outdoors.

Being immersed in holy water in lieu of his regular water would kill him as it would 1) prevent him from restoring his powers (per the draculian need of soil) and then 2) be akin to a lobster boil to severely damage him and since you have already interrupted his rest, he would be unable to regenerate the damage as fast. The electricity would have similar effects in effecting his body as again it would have damaged him during his rest.

As for his Secrecy, he does not advertise his presence frequently, preferring to the shadows in manipulating the UK overtime. Hence which is why he and Magnus clashed at first. His method leads to domination, Magnus' leads to destruction. Bedivere believes in vampires being superior to humans and thus rightfully should rule over them as a farmer may his chickens. But in his long life, he had realized the age of kings failed because it did not establish the despair and hopelessness necessary to subjugate humans because they did not believe in the superiority of other human beings. If the chickens were ruled by a rooster, they merely replace the rooster with another. But chickens are ruled by a farmer. Yet farmers do not slaughter chickens to rule over chickens, they hatch birds, and breed birds, domesticating them for years to become what they are. For Bedivere establishing a vampire kingdom does not begin with a great war, but rather shadowy manipulation such that one day Humans would wake up and find themselves unwittingly serving their vampire overlords. This is his mantra. But if a chicken does get out of line, it would be best to cull what when needed, but not the entire hen house.

So anyone one blow that gets through his armor will instantly kill him?

That's like a vampire wearing full body armor made of the same material to prevent getting staked XD. 
Which seems reasonable, as they do not need the mechanical strength to use such a suit.

There is actually a comic I saw once where a castle had these clocks which were off set by a few hours to trick the protagonists into thinking it was 8 o'clock when he would be facing his vampire adversary who came out with a breastplate to prevent him from being staked.

After the evil laugh, the protagonist pulled opened the drapes to reveal he actually reset the clocks due to day light saving's time was ending to show their actual time.