Re: teaching writing an invitation letter in formal style
My only change, and I'm sure it was just a typo, is that it should be "able to come" not "able to came." Although I am confident Grumpy knows it's a typo, I didn't want someone else to be confused by it.
Also, in the US, we do use the . after Mr. and Mrs. so if you are writing to an American audience, use the period.
I completely agree that statements like "It is pleasing to inform" are not at all necessary to add to the formality and, in fact, are incorrect in your "correction."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.