#1
i think i first developed this habbit from playing the flute

when i play legato passages i often drop the first knuckle of my index finger to the edge of the guitar neck for support.

i DO keep my thumb under the neck. my fingers perpendicular to the frets (well nearly) and play on the tips of my fingers.

i do NOT grasp the neck with my and or move my thumb up around the neck.

now if that is bad technique would one of two alternatives be acceptable

1) to simply dip my knuckle down but not touch it to the neck

or

2) put my thumb against the side of my index finger (while still keeping my thumb in proper position and not touching my index finger the the guitar) and use the thumb as the contact point for the support rather than the guitar
#2
yes... I'm not sure which of the alternatives you really want. "Proper" style for legato with the index finger varies a little bit depending on where you're at on the neck, and what you're playing. This is definitely easier to teach/demonstrate in person...

You don't want your knuckle touching the neck, that's for sure (unlike a barre, you don't want to roll it back at all). You want your thumb behind the neck, (usually) right around the apex of the back curve, directly from your middle finger.

That said, flattening your index finger out generally isn't bad, but it's not necessarily good either.

As for your thumb, the correct way to use it is as a stability/control point, or guide, and not as a clamp (you should be pulling back from your shoulder/elbow for counterpressure). A good guide for your thumb is that it should be the reference point of contact for the center of your hand, and shouldn't be under a whole lot of pressure. Your thumb is really just there to stabelize the back of your hand, not your fingers.
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#3
do whatever's comfortable, i let my thumb ride up on top of the neck and my fingers fly every which way and if it's messing me up then i guess i'll figure it out eventually