About Germany having difficulty smiling, I had seen times where he smiled in the anime in episode 3 of Axis Powers, in the second part of hetalia of the dead (ep 18 of the Beautiful world, while italy was sleeping), and in some parts of Hetalia Paint it white. Looks like that Italy would be one that can make Germany smile.

We don’t take the anime
as canon material. You go to the primary source of material for that – the strips
or any character notes/ blog posts that Hima shares.

Anyways, the scene from
Hetalia of the Dead that you mentioned is added. It didn’t happen in Volume 5,
where Hetalia of the Dead pt. 2 is located.

As for episode 3 of Axis
Powers, Germany isn’t smiling because Italy’s there. Germany’s smiling because
Italy’s not there given how much stress and annoyance the latter caused him in
WW1.

That said, Germany
smiling in this episode isn’t canon in the strips. This is the same scene from the
strips = no smile, but otherwise the same content. Again, he’s glad that Italy’s
not there. Problem is Italy shows up anyway.

As I mentioned in a
previous post, the only time we see Germany widely smiling is when he’s drunk.

He comes close to
smiling when he feels elated, though.

Ex: His love for
machinery. Here, it’s implied that he’s smiling, but it’s not shown. It could
just be something close to smiling too. It’s left open-ended.

Ex: He gets close to
smiling when he goes sight-seeing in Italy. He loves it there.

However, you are right
in that Italy does cause Germany to smile. We see that happen right before the
two of them go off to war in WW2. This is the most explicit example
of him smiling with full conscious awareness.