1) No, see above. You are already in astral form.
2) Depends. It doesn't go into it in the spell and it will depend largely on what the GM decides for his campaign. Things like "Silver Swords " which aren't just any 'silver sword' but a particular type of sword belonging to a specific Astral dwelling creature that due to copy write/not part of the OGL stuff (I believe) doesn't 'exist' in PF (i.e. Githyanki iirc). In general a silver cord is not going to get randomly or easily 'cut' and I don't recall any spells that specifically sunder the silver cord though I'd hazard to guess I'm forgetting one or two that might. There are not any in the CRB that I am aware of.
3) The part below in italics (which I added) is the most pertinent part in the rules I believe.
Glossary CRB wrote:
Effect Spells: Most effect spells summon or create something and are not subject to spell resistance. Sometimes, however, spell resistance applies to effect spells, usually to those that act upon a creature more or less directly, such as web.
Spell resistance can protect a creature from a spell that's already been cast. Check spell resistance when the creature is first affected by the spell.
Check spell resistance only once for any particular casting of a spell or use of a spell-like ability. If spell resistance fails the first time, it fails each time the creature encounters that same casting of the spell. Likewise, if the spell resistance succeeds the first time, it always succeeds. If the creature has voluntarily lowered its spell resistance and is then subjected to a spell, the creature still has a single chance to resist that spell later, when its spell resistance is back up.
Spell resistance has no effect unless the energy created or released by the spell actually goes to work on the resistant creature's mind or body. If the spell acts on anything else and the creature is affected as a consequence, no roll is required. Spell-resistant creatures can be harmed by a spell when they are not being directly affected.
Spell resistance does not apply if an effect fools the creature's senses or reveals something about the creature.
Magic actually has to be working for spell resistance to apply. Spells that have instantaneous durations but lasting results aren't subject to spell resistance unless the resistant creature is exposed to the spell the instant it is cast.
Basically the first time you attempt to move into the area of the Dimensional Lock the you make an SR check. If you succeed you can ignore the spells effects. At least that's how I read it and would run it. If you attempt to Teleport make the check, if you fail the Teleport fails and you have used that usage (memorized, slot, SLA or whatever) or succeed and have fun ignoring that casting of Dimensional Lock.
