When your condition developed into your radioscaphoid and arthritis was diagnosed by your GP and you were off for a period of 7 weeks were you referred to OH Assist for a report on your condition and if so what was their recommendations in relation to your condition being covered within the equality act and any rehabilitation period offered or physiotherapy.?
If so then the manager should have followed the guidelines requested by OH Assist and any deviation should have been bounced back to OH Assist that the management usage of you is aggravating your condition and a 2nd stage grievance would have been in order to keep your delivery manager in check.
Due to the nature of the work your wrist would be in continual use and it would be difficult to find a niche for you in a function where the use of your wrist would be limited other than using your hand that is not effected and I am assuming you do not want to go the IHR route if it was offered to you.
Never the less the work allocated to you at present cannot continue as it could have a detrimental effect for your proposed operation in April for a full fusion and this needs to be brought to the attention of your H&S representative.
You have a long service record with the company and I am also assuming that apart from your present condition your absence record is reasonable and would it be possible to refrain from working until your operation in April where the prognosis of your condition may be favourable and eventually a return to your normal duties and if you can support this from your surgeons report.
The usual crap you get back from management is to use your good hand more often although this might not be your natural hand and would certainly slow you down considerably as you have indicated but my preference would be long term sick until the further operation in April and raise this point with H&S representative and a further referral to OH Assist.