Help please....

I have 2 Amex 2-4-1 vouchers to use

I need 2 Business class seats from LCY to Ibiza returns, both in September 2015 and June 2016.

The outbound in September is available but the return is not.
Returns to London are available to early June but the LCY flights don't seem to have any availability at all.

1) Should I book just a o/w to secure the outbound in September and wait/hope for the September return to become available and then call to get it added - how much would this cost?

2) Should I book the outbound in September with a return in June (to LHR/LGW) using one of my 2-4-1s. Then hope the return in September becomes available and change the return leg when it does - how much would this cost?

3) Should I book legs 1 and 4 (albeit leg 4 would be to LGW or LHR) of a nested ticket now and then book legs 2 and 3 later?

4) Any other sensible alternatives?

I think my current preference is 1 assuming the cost of adding the return is not too high.

Thanks