Don't think you need me now as most of the questions have been answered but I couldn't resist this thread.
From the top:
Scoliosis can make epidurals both more difficult to site and partially ineffective.
In general if a junior anaesthetist (junior = not consultant) is having trouble doing a procedure they would call a consultant/ more senior person to see if they had any more success. V occasionally this isn't possible.
Sadly seniority doesn't always produce success - In the past month I 've seen two patients in whom we just couldn't get a functioning epidural (despite consultant involvement). Some units offer Remifentanil PCAs as an option for this eventuality.
As BB has said spinals and epidurals work by putting local anaesthetic into different places in your back. Having difficulty siting a epidural doesn't necessarily = difficulty siting a spinal on a future occasion. Because of the differences in where the local anaesthetic ends up scoliosis doesn't generally produce a unilateral (one sided block) after a spinal.
If you are worried then it would probably be useful both for you and the duty labour ward anaesthetist on the day you are in labour if you asked to be referred to the obstetric anaesthesia clinic now(even if your hospital doesn't have a formal clinic one of the consultants should be willing to see you).
BB has also explained why sometimes general anaesthesia is required in an emergency. Bascially it all depends on how 'distressed' your baby is. It is much quicker to give a GA than do a spinal (10 minutes vs. 20mins).
However if it looks like there is time then you should be offered a regional technique (spinal or epidural top up) as we know that people like to be awake to meet their baby.
Sometimes even people who have epidurals in for labour end up having GA sections owing to the pressing need to deliver their baby.
Hope this clears some issues up.