I included my corrections in brackets or I re-wrote them. Can you please check them? Thank you.
1)Furthermore, she thinks, the children love it and it keeps her husband good, because it is just three hundred meters away from his library.
2)Furthermore, she thinks, the children love the house and it makes her husband good, because it keeps him three hundred meters away from his library.
3)In addition, she returns to the thought of the chairs, and decides they could bring some from their London flat, together with one photograph and some books.
4)Correction: After that, she returns to the thought of the chairs (or thinks back to the chairs?), and decides they could bring some from their London flat, together with one photograph and some books.
5)When she thinks about the books she reminds (she believes) they are growing up (grow) themselves (by themselves) because she has no time to read them.
6) External reality is the subject of her flow of thought, but in the reading the narrator deals with Mrs Ramsay internal thoughts.
Correction: External reality triggers (?) Mrs Ramsay’s flow of thoughts.
1. The first comma is fine, but the other commas are not needed.
2. Still one comma that's not needed.
3. -- corrections in #4?
4. After that, she thinks about the chairs and decides they could bring some from their London flat, together with one photograph and some books. [YOU REALLY NEED TO CLARIFY WHO "THEY" ARE.]
5. I'm not sure I understand this one. Maybe this? ~~> When she thinks about the books, she remembers that the children are growing up [BY THEMSELVES???] and she has had no time to read them. [I THINK "BY THEMSELVES" IS UNNECESSARY.]
6. External reality triggers Mrs Ramsay's internal thoughts.