.
The words "...of Thy mercy" seem to have been added by
popular acclaim, since the words as given, "...most in need,"
leave open the object of the need. "In need of what?" is a
legitimate question. Certainly mercy is one possibility, but in
point of fact, mercy only applies to this life, and when you
die, you are then facing God's justice.
The second version is making the rounds on the Internet for
whatever reason, but I asked Fr. Gruner about it a year
ago and he replied that it has no basis in the original
Message of Fatima, but are perhaps it's an outgrowth of a
reaction AGAINST the popular accretion that adds "...of Thy
mercy," or now you'll hear, "...of Your mercy."
Actually, in regards to the Blessed Trinity, "Your mercy" is not
entirely inappropriate, since it's directed toward three Persons.
In regards to the second version, it's not theologically
problematic, other than the "pity" aspect, for the souls in
Purgatory suffer according to the infinitely perfect justice
of God, which cannot be in conflict with His infinite mercy.
So God isn't going to be arguing with Himself over whether
or not certain souls are to be released from Purgatory any
"sooner" than what was established the "moment" they
first arrived there. You might wonder, well, what good does
it do then to pray for them? But God knew whether you
were going to pray for them when they first arrived. He
figured in all the future prayers for them when He set
their sentence. If you 'change your mind' and suddenly
start offering Gregorian Series of Masses for someone,
you are only doing what God knew you would do from
the very beginning. So you can ask God to have "pity" on
the souls in Purgatory, but what you're really doing is
saying that here you go, praying the prayers that God
has always known you would pray, so you are fulfilling
your own destiny, as it were, and that of the holy soul.
But in answer to your question, I generally don't say "...of
Thy mercy" at the end of the Fatima Decade prayer. And
I've never used the second version. I've read it, and
thought about it, and asked questions about it, but I have
not used it.
When it comes to the form the words take that we say
out loud, especially in groups, it is important that we all
say the same words together. It promotes a unity in the
Faith that is beneficial for everyone. We have had some
terrible problems in centuries past when prayers were
changed, even by one word -- even by one letter!
When one Pope added a paragraph to the Canon of the
Mass in the middle ages, he was almost run out of town!
And that was a part that somehow survived -- it's called
the Hanc igitur.
So, prayers do change over time, but there has to be
a very good reason for them to change. IMHO there
was no good reason to add "...of Thy mercy" to the Fatima
Decade prayer. Maybe there are souls most in need of
something OTHER than God's mercy, for example. Maybe
His mercy would only harm them. If they are very dug in
to their error or heresy, God's mercy would only push
them further into their heresy because they would think
that everything's just fine, so why worry about it? What
those souls are in most need of is a serious WAKE UP call!
And lest you think that's nice for someone else, it might
also be ESSENTIAL for YOU! In these days of Modernism,
the infection is so widespread it's in the air we breathe,
so how can we escape it? It takes a positive effort to
remove from our habits and patterns of life those things
that Modernism puts there.