Q, thats what I was planning originally but forgot the downspout from the drainpipe runs all the way down that corner and therefore think it rules out the corner mount
All down spouts typically do that on a corner as they lead down to a drain. The only time they don't is where they join up with another gutter or pipe. It sounds as if you might need a tradesman to assist with it's removal if you go down that route. A free hanging downpipe should't be too difficult to remove as they simply pull off, but one into another pipe or a drain socket at the bottom so there's no free end top or bottom would be harder to remove. There's also a risk of disturbing the guttering if not careful.
Also, note, 20ft is very high unless you have other cameras at @ 7ft covering the view close in. You'll get neither a usable face shot nor any footage at all from close under the camera at that height. You might get facial shots of people in the street from long distance where the angle is shallower, or coming up a long drive at long range, but that's useless without anything to supplement it showing what happened when they got close to your property. So note you need other cameras to supplement.
The mounting issue here is going to be the clapped wood. Underneath there somewhere is a wooden corner post for your house that the clapped boards are nailed to, but the issue here is the clapped wood is essentially not very thick and thus not particularly load bearing as screws through it will tend to go into air behind leaving only a small thickness of wood for the weight to be levered against. They're probably going to be a soft-ish wood as well. The wood is essentially just a decorative skin, assuming I understand this construction properly - we don't use it in the uk. To that end, a corner bracket makes sense as it spreads the load and should allow at least two of the screws to screw into the corner post with 2 stabilising probably through the clapboards, although you're going to want long screws to penetrate through the clapboard and well into the corner post near the corner.
From the Wikipedia, this is partially exposed clapboard construction as I understand it, although there could be variations. As I said we don't use it in the UK as we have little to no wooden housing:
Just throwing an idea out there without necessarily saying it's going to work if a corner bracket is impossible, (in other words it's at your risk and it's on you to consult experts for relevant advice), one alternative to a corner bracket would be to spread the weight and create a firm base by screwing an intermediate board vertically onto the finishing board, between the corner post and the camera. Given the weight of the camera, to prevent it from pulling the screws out I'd tend to use a tough hardwood, maybe something like a white Oak, although I'm sure a lumber specialist could give better species specific advice with regards to screw pulling / grain compression / twisting strength. As you'd only want an offcut, you might be able to pick something up for not too much. I doubt with a heavy camera a softwood such as pine would be strong enough. Hardwood also makes it more rot resistant although I'm guessing in any event you'd probably want to paint it to match the facia. The advantage of attaching a board is you can probably use much larger screws to attach the intermediate board to the corner post, than will fit through the bracket holes on the camera bracket itself.
Don't know if you have any design plans for the house, but these will probably tell you the size of the corner posts which might make it easier to know where you can screw safely (too close to the edge and you'll split the wood). It's also handy to know wire and pipe locations, although these might not be on plans!
I would suggest you could maybe use something like a piece 12" by 4.5" (to match the trim board width) by 1.5-2" and screw the board into the corner post in maybe 6-8 points (1 towards each corner (you don't want to be screwing into the edge of the corner post as you could split it - hence the importance of ideally knowing from the plans how thick the corner posts are (you'd also need to allow for the cladding thickness on the outer side bearing in mind the outer house edge isn't really the out edge but the corner post edge + cladding and / or finishing board)) and 1-2 screws down each side, again inset by an appropriate amount, but equally ensuring there's enough space to fit the camera bracket screws between the securing screws) before screwing the camera to it. I'd also use self cutting screws to reduce the chances of splitting anything. Again knowing the corner post dimensions and thickness of trim boards and clapboards will aid in selecting screw length. I'd suggest you'd want to be well into the corner post though with strong screws, probably by as much as 3/4 of it's depth, depending on it's dimensions, as you don't want the board coming free. I'd also fit a safety chain to a separate screw eye screwed into something other than the intermediate board to allow for it coming free (you could possibly loop it around your downpipe bracket here). I'd then screw the camera bracket onto the intermediate board using screws long enough to go all the way though the intermediate board into the wood behind as this will provide extra security even though the hardwood provides most of the pulling resistance. A hardwood with a good strength against compression and twisting etc, should resist against the bracket screws pulling themselves out. Another option could be small coach boats though the hardwood from behind into the camera bracket. Strength here will then depend on the thickness of the intermediate board / it's wood strength. Obviously this is how I'd consider it as an amateur. if in doubt, consult a joiner who can probably advise better. I'm only tossing ideas out there not recommended solutions.
)
