Moving from ShPL to SSP, is it a good idea?

My husband is off on Shared Parental Leave, now into the unpaid portion, while I am returning to work. However, since going off, he has injured himself in a way that would make it impossible for him to do his manual job. He is well otherwise but incapacitated physically. Obviously this is a right pain when it comes to childcare but that's not what I'm asking about.

He's due to return to work at the start of September and whilst we're hopeful he'll be able to walk by then he will still have lots of physio to do so unlikely to be at full fitness by then.

Given SSP is obviously more money than nothing, is there any benefit to him ending his Shared leave early and getting a sick note? Or is there a risk that his employers could come up with some desk based duty for him to do and therefore force him to come in, leaving us scrambling to cover childcare? He can think of a couple of things he could do sitting down but certainly not a full job's worth. We're aware SSP runs out after 28 weeks but all going well he should be able to do his actual job by then (although likely still ongoing physio).

And with his SPLIT days, we know they're voluntary but is that voluntary for the emplpyee or the employer? For financial reasons we'd prefer he was able to do them, so would his employer have to find something he can do if he wants to do a SPLIT day?

TIA