You don't need a lens behind the diffuser, in fact you really don't want one at all because that will introduce an angular-dependence to the meter's response. You don't need "all" the light from the diffuser, just a constant fraction thereof. Ping-pong ball is perfect unless you can source a white acrylic dome, which will be a bit more transmissive.
Certainly you can make a light-triggered circuit, but photoresistors are notoriously wonky and variable in their output (temperature dependent among other things) so it won't necessarily trigger at exactly the light level you want. It'd probably be OK for a B&W shot if you aim to overexpose a little, but I wouldn't trust it at all for colour. Some experimentation would be required!
A (much) more accurate option would be to use a phototransistor, i.e. the same devices present in a real light meter. IMHO you need to admit that what you're building is a light meter and since you have a microcontroller involved, actually build a proper light meter that will compute exposure. That would allow you to (assuming B mode on the shutter) accurately set the exposure time, or if the shutter time is fixed, the light meter can wait until the measured level is exactly the required level. Beware though that phototransistors are very IR-sensitive, so you either want a hot-mirror to cut off radiation below 700nm or you need to accept that you will have an exposure error that is dependent on the time of day.
The other option is to just build a timer with your micro. If you know what time of day will have the lighting you want, a timer will suffice and is much, much easier to build.