My 1997 175 EFI Mercury came with a pull cord in the pouch. Battery only needed to have enough voltage to run the fuel pump. Like you, I tried it. Surprisingly easy. Motor started right up. Never needed to use the cord again.
I would guess that your motor has pull start capability. If so, no harm no foul if the start battery goes dead.
Having said that, I see no problem in having two batteries. One for start and one for the house. With a VSR/ACR/Combiner/Isolator/Etc. between the two.
The alternator on your model...
Many folks have their Mercury motor starting batteries connected to a house battery via a VSR/ACR/Combiner/Etc with no ill effects so that the alternator charges each battery according to its own needs.
Some Mercury models use two voltage regulators where one charges the start battery and the...
Four strokes have been replacing two strokes over time for quite a while now. Less polluting.
EFI models have been replacing carburetted models over time. Less polluting. Less problematic.
Use the link above to get to what Mercury has offered over the years.
Why does it matter?
You know the mix ratio is correct by confirming that the oil pump link rod is set correctly. According to what the service manual says.
When the motor is tilted up the inlets to the carburetors is facing downward. Any fuel oil mix that is in the carburetors can drip downward.
All I think is that the propeller shaft picture in the OP's post looks more like an LF225TURC propeller shaft than it looks like an F225TURC propeller shaft.
Not unheard of for an LF225 gear case to be installed on an F225 motor. Or vice versa.
