1. Branch/Vines/Thorn/Rosebush spells (FA p.49) have a "Sustained" duration, so the plants are temporary magical constructs that disappear when the spellcaster stops concentrating on them. I'd rule that they anchor to any surface as if they grew out of it, but there'd be no disruption or damage to the surface from roots and such after the spell ends. Comet (FA p.50) doesn't summon a literal comet from space, it generates a magic ball of burning earth (i.e., acid and fire damage) above the target. In this case, "above" would be limited by the caster's line of sight. Indoors, it would seem to drop out of the ceiling.
2. Fluff text notwithstanding, I'd say the "comet" explodes when it reaches its target, according to the rules for indirect area spells (SR5 p.283). If it helps, meteors have been known to explode in midair when they superheat (look up "Tunguska event" on Wikipedia).
3. Living beings in the material world don't inhibit the passage of astral forms in astral space unless they're dual-natured, and magical effects in the material world don't cross over into astral space (see "Astral Projection," SR5 p.313). So no, the plants wouldn't inhibit astral movement.
4. Yes, the plants are magical constructs that vanish as soon as the spellcaster stops sustaining. The spell duration would have to be "Permanent" to create real plants that remain afterward.
5. An area-effect spell targets everyone in the area. You'd need to use Spell Shaping metamagic (SR5 p.326) to exclude individual targets from an area spell.
6. Yes. With four hits on your Spellcasting Test, Multiply Food (FA p.50) would turn one soyburger into forty.