Your point CA (if each is sincere and believes in God?)
JW's believe other may be sincere, but if they don't serve Jehovah and attend the Kingdom Hall, they are wrong and will be destroyed by Jehovah.
I think the biggest concern of the JW directors (the Watchtower) is that, in the long run, the JW has a good chance of being pulled down (out) of his/her faith by the non-JW. And a Bible verse backs up this philosophy 1 Cor 15:33 (a little leaven ferments le lump). Now that I have left I believe many of their quirks, like not celebrating holidays or associating with non-believing family or co-workers, are actually imposed to keep their members WATCHOWER-TRUCKING. Unfortunately you could never convince a practising JW on this point.
The favourite Bible verse used by the JW to impose JW same-religion marriage is 1 Cor 7:39 'get married in the Lord only' translated 'in the TRUTH (JW's) only'. They also use 2 Cor 6:14 and call the union of two different animals under the same yoke a bad match.