Reiver,
Why A/THR reverts to speed if FDs Off?
Simple, if A/THR is ON and you switch off FDs at this point the A/THR doesn`t know what`s going on in your mind, and since it is still engaged it reverts to speed and maintains present speed. If you raise your nose it will increase power, if you lower it, it will decresase power. At this point you can do whatever you want and it will always try to keep speed and keep you safe.
Now on takeoff without FDs, I`ve never done it, but you have no A/THR at all, but yes there will by indication of MAN TOGA or MAN FLX, the MAN part stands for manual and it means that thrust is at its respective detent. Also on normal takeoff with FDs at this point A/THR is only armed and will become active when thrust is reduced to CLB, the diference w/out FDs is that when you reduce to CLB A/THR won`t engage and you fly it like a C152!