How involve are the earlier magical girls like Red Rose or White Soul in the World War 3/The Grand Crusade?
Not very. They mostly tried to stay out of it because being a magical girl was still a new thing to them and didn't know exactly how strong they were in comparison to the weapons of the time. It wasn't until near the end when the Grand Crusade actually kicked off to unite the world that MGs finally got involved.
Incidents of them fighting Islamic terrorists or national armies?
Mostly terrorists, psycho cults, mass murderers, and minor warlords. They were considered little more than a hoax until the Crusade.
How they are viewed in those early periods?
As I said, they were considered little more than a Hoax for a long while before the Crusade hit. Urban and local legends that cropped up every now and then that no one could really put names or faces to. A number of countries actually went over the top in dealing with possible Magical Girls and tried arresting suspected ones, which really didn't help matters.
A lot of political leaders flipped their shit, caused mass panic in dozens on countries and a Five Year old Girl did something that was outright impossible and got it posted on You Tube. Lots of Unreasonable People being Unreasonable and shit spiraled out of control faster than you could say 'Nazi dat coming'.
The funny part is Germany was trying to the voice of reason for the first five months of the War.
Did any A.P solder had good working relationship with the others races? Like fighting back to back, don't mind sharing a meal etc?
Yes. Quite a few actually. Commander Mason is one of them: remember the Angel from a few updates back? They're the best of friends.
And knowledge on Half-Angels/Half-Elves had to come from
somewhere.
Can you give us more details about how humanity discover the LDC, how the other races found out about humanity?
Not long after the end of the Crusade people started talking about colonization again. Mostly because of a baby boom.
They stumbled across the ability to open portals to different dimensions by complete accident when a guy researching into new energy sources nearly blew himself up because he watched too much SciFy. For a year or so after that, the still-new Akashic Pillar investigated what was to later be named Hope. Once colonization started and the first city Elysium was built they were introduced to the greater LDC when an Elf survey team portaled right into Elysium's outskirts and nearly gave a poor construction worker a heart attack.
The Elves, once they worked out a few things, let the Fairy Realms and Heaven know about Humanity. Members of the Unaligned Front heard about it from them, and through them the Unified Darkness heard about it.
As stated a while ago, we were soo giddy about other sentient life nearly nobody could really take us seriously, so we quickly built a reputation of being awe-struck moe blobs. The Fairies loved us.
Also more details about second fall of Rome?
The Second Fall of Rome was when the closest Demon Lord to Earth from the Thirteenth Hell decided they were tired of how much energy they had to spend to get their forces to the front lines. Earth at the time, was situated right on the Unified Light's battle lines and seemingly undefended. Making it a highly attractive target to take and hold for the War effort.
So, he took the Second Legion of Hell and opened a portal straight from his holdings and right into the Roman Coliseum.
He had his forces sack the city and kill everything that moved at the time, wanting to quickly turn it into a beachhead to further his invasion of Earth, and roughly 98% of Rome's population was slaughtered.
Local military forces tried fighting back but at the time magic was still new and we had yet to really get into the swing of using it for any kind of military purposes so what forces managed to respond didn't do so well against the numbers arrayed against them. Once the initial 'counter attack', if you could even call it that, was beat off they moved to invade the rest of Italy.
But by the time they managed to do that, Italy figured out what was happening, flipped their shit, and counter-invaded in mass. The local forces beat them back into and through the gate, and Humanity being unreasonably livid from the demonic attack tossed a nuke through the portal as a 'Thank you, but Fuck your everything' gift.