Full pay and NSESA

Not met this one before.

Client has been off sick 5 months. Entitled to contractual full pay 6 months then 6 months half pay. Has currently received SSP topped up to full pay for 5 months.

Employer has issued SSP1 saying SSP ends 30/8/24.  Client has negotiated extension of full pay for 3 months.

SSP1 accompanying letter from employer says:
if you are on half or nil pay - you will keep all monies paid by DWP, providing the total of both amounts do not exceed your annual salary.
if you are in receipt of full pay then all monies received must be paid back to us by method attached. (Following the sentence above I am taking “all monies received” to mean NSESA paid by DWP although the employer needs to clarify this)

Employer is also requesting copies of letters from DWP confirming receipt or refusal of DWP benefits so is forcing client to apply for DWP benefits.

I am thinking that the key issues are that full pay as a voluntary payment is overlapping with NSESA and could potentially exceed annual salary if both are received until the financial year end. (Unlikely based on the prognosis but employer might continue to pay longer and could cause a mess with DWP/HMRC/employer and estate after death). As client is SR1 (not aware) then no issue with going into LWRCA/support group and so money will go up.

Is an employer allowed to ask you to repay the equivalent of the NSESA if they are continuing to pay voluntary pay?
Are they allowed to force you to apply for NSESA if there will be no actual financial gain from it for yourself, and rather a lot of admin?

Thanks in advance for any replies