I can't help you with the push fitting, but I'd say you can probably do without it if you have to. I almost used a quick-release air hose with my set up, for ease in changing tanks, but in all honesty, I just remove the whole primary regulator when I need to take it out, leaving the regulator connected to everything else. I was worried my connection would slowly leak, which is an unfortunate thing to find out when you go for a cold draft.
Regarding your out-pressure from your primary reg, there's no need to have it that high. It only NEEDS to be the pressure of your highest-set secondary regulator. I have mine set around 30 PSI, with my secondarys set from about 25 down to 5 PSI. Except for the very first time it's hooked up, it'll never have to pump hard enough to warrant 80 or 100 PSI, and even then it's more a matter of waiting a few minutes while the system equalizes... Plus you're getting into the danger zone with that high a setting, which leads us to your ball valve...
With your ball valve theory, yes... when you turn the pressure off to your secondarys, you'll have a little stored pressure in the lines. You can pull on a keg pressure valve and release it, or, assuming it's not set at 100, just disconnect it without any precautions... it'll hiss for a second, but that's it. The tube will have a high psi but low volume.
Oh, and if you disconnect from your primary, with nothing on the keg end to also seal the pressure (I have ball/check valves exiting every secondary reg), you'll instantly drain all the co2 in every one of your kegs, leading to a very messy situation, very rapidly. Probably some ruined secondary regulators as well. Northern Brewer has some good ball/check valves. A good idea regardless.
Make beer, not bombs.
(...and not bottle bombs either.)
Primary Fermentor - *empty*
Primary Fermentor -
Secondary - Mystery IPA
Bottle Conditioning -
Keg Conditioning -
Draft: - Apfelwein, Belgian-style Dubbel w/ Grains of Paradise
Bottles:(and still in stock): FIRST AG - (I)IPA 4.1.07, PK's Warm Spring Marzen, Summer Cerveza -BANG, PK's Winter Spice II, Espresso Stout, Indiana Pale Ale, PK's Chocolate Bitter II, Porterfield Porter, and New Year's 2006 remnants.