The official battery is really no different than the Duracell CR2025. The factory welds the tabs onto the surface of the battery and then wraps it in plastic so that the terminals do not come in contact with anything else. You have the correct polarity on the battery; red is +, black is -. About the only thing I can think of to exclude the battery from being the problem is that your soldering was not 100% effective. When you measured the voltage on the battery, was it after you soldered on the wires and was it directly from the tiny contacts of the connector? If you didn't measure from the end of the connector, there is still a chance that the wires are not making good electrical contact to the battery. The laptop seems to think that the CMOS battery is not connected. If 3V is measured at the tip of the connector and it's wired with the correct polarity, then that appears to say that the motherboard has a problem.
Also, it is generally not recommended to solder wires directly to lithium coin batteries. There is / was a chance of applying too much heat and having the battery explode