[Raynor]

Originally Posted by Beanamir of Gondor

Morgoth had three Silmarils in his possession for quite a long time, and he didn't just wander into Doriath and attack Menegroth.

I believe it has already been mentioned before that Melkor feared for his body, that is why he avoided combat.

It's a lot easier for one soldier to slip across the lines than it is for an army to march in. regardless of which war we're talking about.

I don't think that this analogy is valid. Defense lines can be crossed it there is a lack of awareness, something I doubt can applied to matters of enchantment. The text is rather specific in that respect (emphasis added):

Originally Posted by Of the Sindar, Silmarillion

Melian put forth her power and fenced all that dominion round about with an unseen wail of shadow and bewilderment: the Girdle of Melian, that none thereafter could pass against her will or the will of King Thingol, unless one should come with a power greater than that of Melian the Maia.

Beren passed because fate was on his side:

Originally Posted by Of the Coming of Men into the West, Silmarillion

And one of Men, even of Beor's house, shall indeed come, and the Girdle of Melian shall not restrain him, for doom greater than my power shall send him; and the songs that shall spring from that coming shall endure when all Middle-earth is changed.

A rather similar case has been made for Carcharoth, with the mentioning that the power of the silmaril was a factor in his trespassing.