I like working from the top down, so when you next need some work done on your roof then get the chimney pot stripped out down to loft level and the roof repaired.
No help to your plan to redecorate the downstairs, perhaps you could remove some of the bulk of the chimney and keep it as a decorative feature.
I had a roofer out and he said it would be difficult to repair the roof because the old felt is not breathable and you cannot get it - so it would be like a bodge job. Adding breathable felt / lining to an old felt.