First meet the 180 day deadline for AC21. I think your underlying I-140 is already approved. So that should not have a problem after 180 days. If not wait for I-140 to be approved. Don't hesitate. Take-up the full time offer, but plan ahead and hire a good attorney. Like Q mentioned, AC21 is seldom denied. The attorneys can wordsmith and add functional roles to get AC21 / I-485J approved.
Q1 & Q2. No. Please browse through
USCIS PM Chapter 5.It explains you the factors that USCIS considers in approving AC21. If the company is onboard, the attorneys can justify "Director" by providing a "Functional Title". As GC is for future job they can even provide a I485J for "Non-Director" position and let you work in the current role till you get a GC. You may have to technically work in that position after getting GC, but there is no way of enforcing that. The only thing USCIS looks at while applying for N400 is where you worked after getting GC and whether you worked there for a "reasonable" time. Maybe you can take on some additional duties after GC to perform the functions in GC/PERM/I485J. *wink*
Q3. Looks like you already have everything. Remember the USCIS adjudicator that is going to review your I485J does not know anything technical about your job. Its like a street peddler trying to sell a micro-architecture. So, as long as the justification sounds reasonable, he is going to approve it. If it were me, I would come-up with AC21 / I-485J job description by wordsmithing to match closely with PERM and give it to your attorney / HR as a start. A good attorney helps a lot.
Q4. I485J serves two purposes. First time, it stands for bonafide job offer. The second time it is for portability. USCIS can ask for any of these if they wish. Recently I saw some RFEs that asked for confirmation that the applicant plans to work in that job. GC is given based on a contract between you and the company. You promise to work in that position after GC. The company promises to offer you that position after GC. That is the whole purpose of I485J process.
Spend some good money and hire a good attorney. Or use the attorneys from initial PERM/I-140, pay them and ask for justification. Nothing works like money. But you are the best judge of the job justification as it was your job.
Good Luck.