We are sorry to hear of your injury and wish you a refuah sheleimah!
Yichud is permitted to husband and wife even when there are longer periods during which they must abstain. There are no restrictions on you and your husband being alone together during this extended time in niddah. Because the two of you be"H will be permitted to each other in the not too distant future, there is no concern of lacking pat besalo here.
It is worth noting that you may be able to immerse sooner than you think. There are mikvaot designed to accommodate women with limited mobility. You may be able to schedule immersion for when you are having a cast change — or even arrange for one for this purpose to fiberglass, which is water proof and is not necessarily a barrier to immersion (if there is space for the water to enter between the cast and the body). Please write back should any questions arise in this regard.