SR II: Invisibility, Spell Targeting, and Astral Perception, Help! Options

[Frieza_Prexus]

So I've been playing some SR II with some friends, and we've been doing pretty well, but after 5 modules an issue has come up.

The players are using invisibility, and I'm wondering if I'm adjudicating it correctly.

My understanding is that you cannot cast a spell at something you cannot see. Astral perception allows you to sense the target if they're invis, but not "see" them. In SR II (I don't know about later editions) they talk about the physical/physical or astral/astral symmetry required in order to "sync up" and hit a target with a spell. Depending upon how it actually works I've got two possible interpretations of how invis works:

1: If your eyes cannot see a target, they are not a valid target. Period. Astral perception gives no advantage other than locating the target for you. This means that blind mages cannot give any form of spell defense or cast any spells that require a range greater than "touch."

OR

2: If you cannot see a target, but can perceive them they then become a valid target for a spell PROVIDED that you have the appropriate physical/physical or astral/astral symmetry (or astral/physical if there's a bridge).

Also, if option 1 is correct, do dual natured critters have any special advantages in this department? I'm about to run Bottled Demon, and I'm trying to plan out Geyswain's tactics. He'll probably just melee them down, but if they're all invis could he not cast a sleep spell on them?

Finally, if 1 is correct, and I choose to house rule that option 2 is correct, what horrible unforeseen consequences await me? I don't see anything going awry provided I keep the symmetry requirement.

Thanks for the help guys.