I thought ahlan was more like 'welcome' in meaning. It doesn't feel right translating it as Hello.
Hello is more like "مرحبا" pronounced as "marhaba"
Quite right. My understanding was Marhaba was Hallo.
Is omitting the يا possible? Is it still the same meaning?
It's possible and is actually quite common in spoken language, but it wouldn't be proper Arabic.
It's good to have that.
What about "Marhabn"?
Loses you a heart if you put Hallo, not Hello