The best advice I can offer here would be to find someone that plays cello (well) and ask them to re-tune their instrument and try and work through the passage in question. I greatly suspect this passage will lead to copious amounts of frustration and head-shaking on the instrumentalist's part.
Strings are used to reading / thinking in sharps because of their
open strings. Scordatura has its own problems to begin with, but
such awkward intervals mixed with asking the player to think of the
strings as all flats is just begging for your piece to never be
played ever after its premiere. Not being mean here, just being
Are you notating what you want to be sounding when they play with
scordatura tuning or are you notating what you want them to play
with scordatura - regardless of how it sounds? There seem to be some
notation inconsistencies here.
Here are some reasons that make your passage extraordinarily
- Notational inconsistency
- Suggested tempo
- Range of instrument
- Scordatura tuning
- Awkward gesture
Cellos have thicker strings, intervals are wider apart, instrument is bigger, and gestures easily done on the violin sound typically less effortless on larger instruments.
Personally, I think this passage is unnecessarily difficult to get the effect that you might be looking for. Again, I strongly recommend you have someone play this so you can learn first-hand. I predict that they would end up omitting your octaves in mm.357-358 entirely and arpeggiating the intervals in the remaining measures in order to make the gesture work.
Asking a cellist to meet all of the demands above, as well as marking the notes tenuto with detaché bowing at the specified tempo is at best going to make those measures sound clunky.
Instead, you may want to look into bariolage with legato bowing indications which might give you the effect you're looking for (not to mention emphasize the scordatura - after all, that's the point of scordatura right?)
Also, it's not necessarily my business, but why would you ask such an awkward scordatura and then mark the key signature so traditionally?