Couple of points.
LPA's don't have to be registered straight away, but for the reasons given, it is very advisable to register them promptly. I suspect a solicitor would want to do it immediately so that he can charge more.
When filling in the forms, all the signatures have to be obtained within a 12 month period (I think they say "without significant delay"). As our children live far away I checked with the help line what his meant, they said 12 months. However they suggested that once I had completed the forms online, our children could print off the relevant sections and sign them.
Regarding the various witnesses etc, they do not have to be professional people (like a passport) just friends etc that have known you for a while.
It does take up to 10 weeks to register. After they have checked the documents, they then have to write to every party concerned giving them 4 weeks to object etc. Given leeway for postal delays this probably takes up 7 weeks of the 10.
I believe you can change addresses etc with no problem, but I suspect adding a new attorney would need a new application. You are able to name several attorneys and replacement attorneys.
Regarding cost, if your income is below £12k/yr or you are receiving certain benefits, the costs are reduced.