Shutter release button:
The fault sounds like it is mechanical BUT this sort of issue can be due to failure in an electronic sub-circuit.
For the reasons expanded on in the next paragraph, you MAY be able to easily access the switch in your camera BUT for certain, in some cameras it is an exceedingly demanding task. There is then the chance that the fault is electronic rather than mechanical and you may not easily be able to identify or obtain the required parts. Having a service manual would help greatly. I once dismantled part of the shutter mechanism of a DSLR to address a problem. Without a manual it took hours and hours and hours and many many attempts to reassemble. The manual explained the arcane tricks which made the task just very hard.
- Accessibility of the switch depends very much on the specific camera. I had a very similar problem with a Minolta 5D "Alphasweet" [:-(] DSLR. The camera could be triggered using an external "shutter release cable" but could be focused but not released using the usual shutter button. I am reasonably experienced in dismantling and repairing mechanically complex and 'fiddly' equipment, but I found the camera to be too annoying to be worth doing myself. While the shutter button was just under the external surface of the camera you needed to progress around the whole camera modules and covers and wiring and connectors and more to finally get to the switch assembly. I entrusted the repair to the competent and value for money care of Steven at Camera Hospital in Singapore. I consider that that repair would not have been possible by other than a mechanically skilled and experienced repairer. I could have done it but it would have taken me far longer than an expert took.
Flash capacitor discharge: I would not be overly worried by the voltage which may be present due to the flash. A likely way of minimising voltage is to charge the flash, remove the battery and then fire the flash by taking a photo with the camera pointing into a large dark space which the flash cannot illuminate well. If it is not possible to tak a shot with no battery, do as above with battery in and then remove battery asasap after firing. Then lease the camera for a few hours but it will not be overly dangerous regardless. Very very very worst case a photflash shock MIGHT kill somebody by triggering eg a heart attack form the SURPRISE of the shock BUT in the very very large number of cases it will just bite you annoyingly and then be safe.
Capacitor will usually self discharge in minutes to tens of minutes. Maybe a few hours if very good quality. Shorting capacitor with a say 1k resistor will discharge it in a few seconds. Shorting with a wire will discharge it rapidly. Small danger of minor metal spatter into eyes if wire used. Ordinary glasses would protect OK. Can be a LARGE bang from high power flashes - 1 k resistor better.
Flash voltage: The "power" of the flash is only somewhat related to the flash voltage, and battery characteristics. A normal flash has a gas (usually Xenon. Krypton in some specialist applications) in a "discharge tube". The gas is ionised so that it conducts electricity and the discharge excites the gas to produce the flash that you see. The actual discharge voltage for even a very small flash is typically in the 200 - 300 Volt range. In addition there is a trigger coild that generates a MUCH higher voltage to initiate ionisation. This is typically in the 1000 - 5000 V range but may be 20 kV in some applications. That voltage is not stored and will not be a hazard in almost any case.