RMH plans- help with chimney lenght, negative pressure, channel layout.

Hello everyone!
My name is Marta and I am writing from Spain.
In September we are finally quitting our jobs and moving to a house in the mountains near the mediterranean (therefore mild climate, it snows some years but not every year, mostly sunny, dry, the wind is a bit crazy though) with our 16 months old baby. Yey 😊
We want to build a RMH and would very much appreciate any help checking if the design seems to be correct before we get to work on it and answering some questions that have come up while reading the RMH guide.
I’ll try to explain myself as good as possible, sorry in advance for my English mistakes.
The house was and old building made of stone to keep animals which was renovated and extended with a new wing made of concrete walls. It has two levels and three chimneys. We bought it like this and are trying to finish de new wing, where the RMH will be (also the future main living area).
Here are some plans and photos that explain better. We are also inspired by this design we found online: Ben's Natural Building: Rocket Mass Heater Design/Build Workshop: Restaurant Selmellà
As the chimney is already in the other side of the room, part of the channel will be only one tube 20mm, is that possible? I mean, is not going and coming back like on the examples on the book.
The chimney length is 2,70m inside and 60cm outside. The stairs don’t have a door and there are two chimneys upstairs, there is also a window. There is a metal door that separates de new wing form the old house. We wonder if we will have problems with negative pressure and options for fixing it. 1. We isolate the metal door ad keep it closed when the fire is on, This Is the simplest option but: does this mean it can never be opened, even just to go through quickly? 2. Add a door to the stairs (this will be difficult due to current layout, but maybe is the only option) 3. Shut down the chimneys upstairs, since they won’t be used in a regular basis. 4. Increase the height of the chimney outside, but how much?
The RMH would be near the stone wall in the middle of the house hopping some of the heat will be stored and transfer to the rest. To your best knowledge, do you think this will happen?
Thank you so much in advance to anyone with the time and knowledge to answer.
Kind regards to everyone reading.