I apoligise for my ignorance for the subject but I have a marine offshore fabrication background and thought I may offer a suggestion that may spark someones thought process so please bare with me. I have reviewed the sketches available of the BOP and riser layout as it is . I know that there are numerous crane barges and or cranage of similar capacity locally, that perhaps that you could attach a cable to the rise rpipe( with rov assistance) at the kink area closest to the bop and with tooling an rov to remove the fastening system of the riser from the bop and pull it up. Youcould possibly remove the riser and associated drill pipe from the bop. this may clear a joint or remove it all together to allow the shears/ram to close. You may have to remove it anyways for the funnel apparattus.