A gap big enough for a coach and (4?) horses would be a very wide space indeed, so this is not so much about finding the opening or getting around some rule. This idiom refers to the damage that would be done by all those horse hooves and the cart wheels. Imagine if someone drove a coach and four horses through your garden- the noise, the mess, the dust, the damage that would result- there wouldn't be much left!
A chink in one's armor is some small gap that makes you vulnerable to harm. You seem to know this, but your post asked for comments on the expressions in bold type.
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