It sounds more like condensation. Warm air is escaping into the attic, where it meets cold air, condenses and forms liquid.
While it could be a leak in your roof, the fact that you're only noticing it at your smoke detector usually points to condensation, otherwise you'd expect to see other signs of water damage (stains on ceiling, spongy feeling drywall, etc.)
You really need to get into your attic to confirm. If it's wet all over, good chance you have a leak, but if there is insulation missing around where your smoke detector is (wires, etc.), then you want to ensure you properly seal the hole and ensure the area is adequately insulated.
Your furnace is shutting down on limit. There can be a number of causes for this.
Start by making sure you have a clean filter in the furnace. A dirty or restrictive filter can cause this issue. Alternatively you could also have a dirty or plugged up A/C evaporator coil which is located above your furnace. This can also cause a furnace to cycle off on high limit, however is uncommon unless you have run your furnace without a filter for prolonged periods of time. Also if you have a high efficiency furnace, there is more of a chance to have the secondary heat exchanger plug up rather than the A/C coil.
You can also have a blower motor issue. The motor can be faulty or seizing up causing it to not spin fast enough to move enough air through the furnace. Another cause may be that you have a faulty limit that is opening prematurely.
The major issue that can cause this problem is a heat exchanger issue. If your furnace is cycling off on high limit, and everything checks out, you may have an internal blockage of the heat exchanger. If this is the case either the heat exchanger or the furnace will need to be replaced. A combustion analysis of the furnace is required to prove this is the issue.
If you happen to have a 2 stage high efficiency furnace you could have a bad ventor motor gasket which is allowing air to be drawn in behind the motor. Big cracks in the condensate collector pan can also cause this but is very uncommon. You can also have the a two stage gas valve high fire solenoid stuck open, but this is also very uncommon and can only happen on 2 stage furnaces. Also if you have a bryant, payne, or carrier high efficiency, you could have a cold spot baffle leakage that is allowing air to be drawn in between the primary and secondary heat exchanger however this is also very uncommon.
If you think your blower is not spinning fast enough you may also have a bad blower motor, or if an ecm motor, the control board may be bad. If its a fixed speed motor and your getting power to it but its not spinning fast enough it could possibly be a bad blower motor. If you decide to change out the blower motor, make sure to get the appropriate replacement capacitor to go with it. I actually did have the same issue on an older carrier high efficiency furnace a few weeks back. It ended up being a bad blower motor, however the motor spun freely and had no indications of seizing or hard starting.
Best Answer
To me it looks like the drain line is plugged causing the condensate to build up in the air handler. Once the water level is high enough it finds a way out.
You know your pump is working but now you need to make sure to clear the line from the air handler to the pump. It is quite common for dust and spores to develop blockages in the tubing and this creates the problem you have.
There is 1 more possibility that a low Freon charge could be causing the evaporator to ice up and possibly water dripping down the side wall is causing the leak but I would be clearing the drain tube (this is quite a common thing).