Small summerhouse letting in water through shiplap

I have a small summerhouse that is letting in water through the shiplap joins, cracks in the wood , wood knots and where panels meet but haven't been filled properly. I cannot access the outside of the summerhouse to paint it again because the pleb that built it before I bought the place put it right up against 3 walls. What can I do from the inside to stop water from getting in? I had planned to seal the bigger caps with Expanding foam, Skia Flex EBT the joins that were just not done correctly and Bostik Aquaprufe Damp Proofer & Waterproofer over the top of the shiplap to stop any water /damp from coming in and adding two small vents to get fresh air in and keep the condensation out. I'm sure this solution wont last 10 years but if it can keep it's self together for 5 I'll be fine with it. I did a mold mildew bioside wash to kill what was in there, and have just gone over it with some ronseal mould and mildew treatment. Thoughts?