Hey there, welcome!
I'm glad Mark's message inspired you. I too am pretty new, but had been primal for six days when he sounded that call to arms
You have some tough circumstances, and I'm afraid I don't have any sure fire answers. One thing I do know though is that every day you eat paleo is a win for your system. So as long as you keep trying you are slowing down and delaying some of the negative aspects of the SAD.
I've never really suffered from cravings, more a question of getting a little bit bored - but that is my fault for not being creative enough, well, that and trying to do this on a budget also in a household that is not 100% behind me. I think you have to treat the cravings with respect but they are where willpower comes into its own. Your on-off method is feeding your cravings. That part can only be beaten by riding it out, that is when you will really really need your willpower. Set yourself a target number of days to be primal and stick to it. Make a seven, or even ten day, menu plan and stick to that. Although primal is not a diet but a lifestyle, I think in order to beat the cravings you have to treat the first couple of weeks as if it is a diet - use the mental toughness aspect, but also know that is finite. In seven or ten days you can have a portion (not stuff yourself until you cant eat anymore) but a portion of whatever it is that you wanted most.
Maybe your family would better understand if you focused more on the wanting to build muscle (and get ripped) side of the benefits of paleo eating rather than trying to explain to them the real benefits of paleo.
I'm not sure how bad the situation is when you say junk food and/or whether they ever have in real food anyway, but maybe one strategy would be to pick your fights rather than not eating anything they eat at all. Of course that depends on whether they ever eat anything real.
Finally, maybe concentrate on the other elements of the primal lifestyle more than the food - attend to the other Blueprint bullet-points - get your daily dose of Vit D, move around a lot at a slow pace (i.e. get out of the house and away from your negative environment), sprint, lift heavy things, sleep well - create a chart and tick off each of these things every day, it may just motivate you to sort out the food part, too.
Anyway, I've have mind-bled onto the page enough. I hope something I said might help.
Welcome! I look forward to reading your journal and seeing your progress