You can call any BA call centre now, and there is 24/7 coverage (although obviously this involves an international call).

If you are booking open jaw, are you planning to book both the outbound and return at once? In other words, is the outbound available now and the return flight is 355 days in advance tomorrow?

If not, you can only book the outbound and will need to call back to book the inbound, and so you may as well just book the outbound online as soon as it is available.

To answer your second question, no you cannot generally change an open jaw online and would need to call (and pay any service fee).