If it simply stopped dead, you'd be looking at the primary ignition circuit. If only one cylinder was not firing, your engine would be running although you'd notice the loss of power. Most people seem to suspect a rusty fuel tank though.

Use of clutch: not that I'm an expert on this, but my reasoning is as follows:

The synchros are used to match the rotation speeds of the transmission input and output shafts before engaging a gear. The output shaft always turns proportional to road speed. The input shaft (ignoring the clutch) always turns at engine speed. Both the rolling car and the running engine have quite some inertia.

If you use the clutch, the only mass the synchro has to slow down is the freely rotating input shaft.

If you don't use the clutch, the masses to be synchronized are much greater. If you force it in gear, the synchro will be strained a lot. If you lightly push it until it slips in gear (when the motor revved down enough to match the revs), you have friction until the gear engages.

Maybe it does not make a difference in reality, but the clutch is made to cope with a lot of friction, the synchronizers are not. I think I know which one I'd rather change :)

Same story for the overdrive if you have one. Some people use the clutch when engaging OD, others don't. But if you want to put the least strain on your internal OD clutch, use the clutch.