So long as the junction box is suitable then yes you can.
I would suggest you use a box that comes with suitable cable glands, as making a hole / using just a knockout will let rain in. (Unless you "cheat" and use the knock outs at the bottom of the box and make a drip loop on the cable.)
As for the cable, plastic trunking is not meant for external use, PVC conduit IS meant for external use, but its primary reason is to give mechanical protection, so unless you plan on hitting the cable, I would suggest so long as the new cable is suitable for external use then mechanical protection need only be an option.
There is a limit of 150W without planning permission, so personally I would use a smaller lamp. Think I have 8W LED in one house and 15W in other both ample for what I want. I have a 70 watt metal halide and never fitted it as too bright, would need sun glasses.
Daisy chain in normally considered as a series connection as with Christmas tree lights and aircraft down lights, this is not really an option in your case, however in parallel yes you can have many lights from the same switch, we are limited by regulations to 16A for lighting, most switches rated either 5 or 20 amp and most lighting circuits protected with a 6A MCB or fuse, so total over 1000W so with 150W limit that is a fair few lamps, since on switch on lights use more power I would say 500W is a realistic limit for a 6A switch to be trouble free.
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