Welcome to the forum. I'll try to help. Bottom line: there was little or no risk of HIV from these commercial sex exposures and your test results show you were not infected. You really shouldn't be worried about it.
Use of plastic wrap as a condom substitute has a long and probably glorious history -- I imagine the first use for that purpose was within one day of the first time such a product was marketed, replacing waxed paper in the average American kitchen. (Waxed paper doesn't appeal as a condom!) Although there has been no research on effectiveness, probably plastic wrap is highly effective. As you surmise, the main problem likely is penile coverage. However, if the head of your penis indeed was covered, you were well protected against HIV, and against those STDs transmitted through genital secretions (e.g., gonorrhea, chlamydia), but less so against those transmitted by skin-skin contact (e.g., HPV, herpes).
Having said that, the other issues are whether or not your partner is infected and whether your negative test results are reliable. The chance she had HIV is very low, probably under 1 in 1,000. But most important, the modern HIV tests are highly reliable, including the method used by Home Access -- 90+% reliable at 4 weeks and virtually 100% by 6 weeks. To your specific questions:
Those comments answer questions 1 and 2. Question 3: It is exceedingly rare that someone's HIV test becomes positive after 6 weeks. I cannot say it has never happened, but people also get struck by lightning -- the odds probably are about the same. Question 4: Yes, another test at 8 weeks would be 100% reliable for sure. Here is a thread that discusses test reliability at various intervals, and why testing by 6 weeks (and for sure 8 weeks) is sufficient, even though many agencies still recommed testing at 3 months: http://www.medhelp.org/posts/HIV-Prevention/-A-Question-on-Testing/show/1347755
Bottom line: There is no chance you caught HIV.
I hope this helps. Best wishes-- HHH, MD