why was העתיד pronounced 'he-atid' and not 'ha-atid' here? when is ה pronounced as 'he' instead of 'ha'?
Ah, the beauty of changing initial letter again... there's a whole set of rules for the he of definition, just like for the vav of conjunction. I could explain if you like but in reality, nobody says it like that in daily conversation, only maybe if they are professional narrators on the radio or something. We say ha-atid.
I mastered those funny rules only to have to unlearn them when I speak to Israelis! It's a shame as I thought they would work with my plummy English accented Hebrew what, what...
I mastered the vav of conjunction and then humiliated myself on a daily basis until I unlearned it.
Let's see if I can recall. When the word starts with ע or ח with the kamatz - עָ, חָ - the definite ה gets a segol הֶ. This also happens with words beginning with הָ, but only if the accent is on the first syllable. Really nobody bothers about this because we don't always know whether the nikkud is kamatz or patah or hataf patah, for example I remember it's הֶחָתול but הַחֲזיר.
l הסברה is more like a campaign to raise awareness, going on a civil term for propaganda.