$300 MILLION in just 6 weeks,
300M$ seems huge to me. My french company in HaTF (december 2005) pays 50M$ in salaries weekly, which fits your scenario. It, however, flies 114 A320s, 377 A140s, and 21 TU-204s. You don't schedule that kind of fleet within 6 weeks. It goes quicker for very larges, but still not to that point. You probably need to schedule 40 or more very larges at once to reach those levels of staff expenses. And you don't get 40 very larges within 6 game weeks when beginning. You can only once you're huge (and then drowning under money, which makes the whole point moot).
When beginning, assuming you already have a nice money flow, you can schedule 3 birds every 8 game days. That's 15 very larges within 6 weeks. That's very below your 300M$ of estimate staff costs. Either you are overestimating the staff costs of planning too early, or you mistook other costs for those costs.
Worse : there are solutions. You can set up staff hiring to manual when beginning, set up your routes without putting them on the screen, of deleting them (without closing) as soon as you like them, and you have no problem. Those solutions may be suboptimal, but they work. And even if they don't work, a properly set up company can rocket start without optimizing on this.
And I don't get this 6 weeks delay. Used planes arrive in 2 weeks, new ones are only available 3 weeks. Where are your 6 weeks? D-Checks? Why do you do D-Checks on such planes?
And for the commonality costs : well, as others said, it's another topic (which I didn't adress in my first answer). If those 300M$ are really comm costs, well, just wait for a few weeks before flying. Once again, this is NOT in relation with the ability to delay slots usage. I'll go even further : if 300M$ of losses during a transition kills your company, then your main problem is not commonality. It's basic weakness. For the comm costs to be punitive you need to be beyond 600 airplanes in your company. with 500 not that big airplanes, I do 40M$ of weekly pretax profit in 2005. That's a 400M$ quarterly operational positive cash flow. I'm still too small to incur such commonality costs. At this level, you should be around 100M$ pretax weekly, something like 1B$ of quarterly positive cash flow, and good reserves.
What your numbers show is that you grew your company beyond sustainability. A single bad event (self provoked, but could be external, as a volcano) is enough to put everything to the ground. You were brittle, vulnerable, and some kind of clever player would have attacked your positions, sooner or later. rereading all the thread an your explanations, my conclusion is : "
you were doomed anyways". This way of working may have hastened your demise, but it was unavoidable anyways.
(That being said, groundbum2's proposals are most excellent - would make life really easier).