Hi,
I can't help you with a schematic and risk a ridiculous wrong solution to a device I do not understand. I am also slow and stupid, I have finally understood the first post. Sorry.
If you think it will work, what better than a voltage regulator like an LDO or just a dirty little 5.1V zener diode as an overvoltage protection device (I'm not good with zeners so expert advice might help if that's a nasty solution here)?
Look at the charger and see the max output volts and output current it can do, and then check the micro-usb source can do the same, if so you could look for maybe an LDO that is extremely low dropout and can handle maybe double the estimated charging current of the phone. Phone chargers are just SMPS and nearly all are 5V, 200 to 600mA, from those I've seen. An SMPS IC might work. You need to know the input voltage needed to the phone and the output voltage from the charger.
All this said, I think besides quick solutions it would be a great idea to sit back and try to get info. about how the phones use charge from the battery. I hope you find a schematic, good luck. You want to power a phone from a supply but with no battery, I understand. I'd guess that phones may have multiple voltage regulators straight off the battery pins or only one main one and a few dependant lower voltage ones off the main one - I'd really tru to get a vague idea of that internal circuitry somehow. Sorry this hasn't been much help.
Silly question: have you tried to hunt down mobile repair technician websites or blogs, if they exist, or Samsung forum or modding forum things, if they exist? Without wanting you to waste time on a wild goose chase, I think some of them occasionally have threads where you can get lucky with a schematic or a helpful thread but that can be very hit and miss and even a frustrating waste of time, you never know though.