Funny you should post this question - my Mom has used this on me also. However, I pointed out a number of scriptural truths to her on that subject. The Devil will keep you busy, distracted, and a number of other things - but he will never make you truly happy. Jesus himself said that the devil comes to steal and destroy - only Jesus came so that we may have life "and in abundance".
So it seems clear to me that if you are truly happy and content, that it is not coming from the devil. Things from the devil are fun on the outset but leave a kind of emptiness and sadness in the end. Things from God leave a good complete feeling in the end.
It's a concept the JW's find hard to grasp.
And I also agree with your assessment that on God's love for everyone. I do not believe that only one denomination has the "truth" and will be saved.