You Said the Electromagnet was 4.4 Ohms and rated for 12 volt.
So 100 mA is FAR BELOW and Damage.
And the 100 mA is NOT Delivering 12 Volts. NO WHERE EVEN NEAR THAT.
If you Amplify that Signal with a Power Amp and use a Matching Transformer, you could bring it up to a MORE Suitable Power Level...
100 m Amp?
Is that 100 Milliamps?
Possibly use a Power Amp and a Impedance Matching transformer.
But getting 33 watts with 12 volt supply is very unlikely.
You could use a Square Wave, Driving a MOSFET to get More Current flow.
There is a Considerable Difference between powering them from 12 volts DC and a Low Frequency AC.
It is NOT so much about the DC Resistance as it is the AC Impedance.
Also are you using Sine Wave, Square Wave or some other Waveform?
Yes it is Possible, But first you need to find a clock that has an option to Stop it and hold the time.
And you can use a Photo detector or a Proximity Detector.
In My Opinion YES.
As to Copper Pipes, I have used this solder for that also, But you Need Some of the Liquid Flux.
Not just the Small Amount that is In the Solder Core.
I prefer a Kester Brand 331, 63/37 Water based Flux Solder.
Than you can just wash the whole board under a Warm water Tap to Remove all the flux from Both sides of the board. Dry it and away you go.
The Diodes are quite Light, But 50 of these capacitors weigh Over one pound.
When wrapped up and Sending these by Canada Post Air Mail, I expect the Shipping cost will be around $17.00.
Payment to me Must be Actual Money, Mailed to me in a Letter.