As a 2nd tour 1630 looking into many of the same questions, here are my answers:
Question 1: Attachment to SPECWAR/USMC
As a first tour JO (non-LDO), you will likely not be assigned to a SEAL/SPECWAR billet. As a reference, check out
www.npc.navy.mil, get a login if you don't already have one and review the intelligence section of the site. If you have problems with this course of action, here is the long and short of it: you must be at least a second tour O-3, within 18 months of your PRD you must make you intentions of SPECWAR known to the 1639 JO detailer, have a letter of reccommendation from both your department head and CO, have scored better than Excellent Low on the past two PFAs (with a letter from your CFL certifying said results), and finally interview with a former/current SPECWAR N2 assigned to you by the detailer.
As far as being attached to a USMC unit, the closest you will likely get is serving as ship's company on an LHA/LHD. There are some IA billets that exist out there and I'm sure that if you're willing to volunteer, someone will take you up on the offer!
Questions 2: Timeline
For IA billets, let your department head know of your interest today, if you'd like.
For USMC ESG billets: Ask the detailer
For SPECWAR: Wait until your second tour to begin the above mentioned process.