Hmm, well in time it looks to be quite difficult. If it is a true cadenza, then it might be more practical to remove the "alto" harmony at that point, and delegate the passage to a single violin soloist. The soloist could bend the tempo ad lib., but the re-entry of the theme should be a tempo.
If its not a true cadenza, then I would suggest keeping it in time, and altering the violin I part.
Hope that helps!
- BE