Personal preference. Both are kiln dried. Horizontal is probably more cost effective for waste but vertical may look more aesthetically pleasing.

I would agree, personal preference

In terms of which way to hang, the direction can also be used to make the house taller or wider so I’d add a picture

https://duffieldtimber.com/blog/cedar-vs-larch-cladding-which-should-i-choose

Planning on cladding top half on front with a 1 MTR return on side & rendering lower half

I‘m not sure I would do all of the bottom in render (particularly if it’s going to be white with the contrast) as i think you’ll just visually split the house in half.

Sadly my photoshop skills aren't good enouh to do a visual for you but I would try and vary the cladding and render similar to what they've done here

Western Red Cedar is more durable than Siberian Larch.

Accoya and Thermowood are also good options.

I prefer vertical slats personally, I think it’s easier to install, more weather proof against driving rain and looks less like a shed or fence.

What type of cladding pattern are you going for? T&G, Shiplap, Board-on-board… Will you be leaving it to silver or sealing with a finish?

Bear in mind you need to create a ventilated rainscreen, vertical cladding will require cross-battens whilst horizontal requires only vertical battens.

I would also research into creating fireblocks within the ventilated rainscreen.

Check out the Timber Decking & Cladding Association, they were super helpful in advising me in the different methods/materials for cladding my garden room. They have free literature on the topic too if you want to jump down the rabbit hole further :-)

https://www.tdca.org.uk/