Yes, you can all play together and just be a squad of 3.
If you bring in your under leveled friend on the higher difficulties it's funny as Hell and a right of passage. Lower geared Helldivers can absolutely contribute to victory in higher difficulties, if they know exactly what they're doing.
Jump in whenever. Fight what you want.
You get a number of missions in a campaign depending on difficulty. The missions are objective based which repeat and cycle in and out. The maps are randomly generated and never feel the same.
Build diversity is actually a thing. You can't take only pistols. You must take a primary weapon, secondary (side) weapon, and a throwable (usually grenade). You get stratagems to call down and they give access to heavier weapons.
I'm basing on this on absolutely no experience in the matter, but I don't think a degree in labor relations is going to be in high demand in another country. Unless you have a legal background or the lawyer accreditation equivalent in that country' s legal system. I say this as a man with an Anthropology degree.
Finding out what skillsets those countries are seeking from foreign workers and accreditations to prove those skillsets is probably step one in emigrating.
As a man that grew up in Texas I desperately wish Canada would let me in and adopt me.
Look at its face. Look at its weird hands. I would say that's a blue horror in a green skin suit, but it's even worse.