The one person who cannot sign the PROB 11 registered copies is the testator as he or she should be 6 feet under at the time! It does appear that the handwriting in PROB 11 sometimes differs for the signature - I don't know the reason but wonder if it is because the wills are copied in a very stylised handwriting and perhaps they altered it to indicate a signature.
A will could be drawn up without a solicitor - especially with the earlier wills. I've come across many where the parish clerk was an obvious support in the production of wills instead. Also, I don't think there were any rules about who held copies. What is still the case is that executors have to turn up and swear to carry out the instructions in the will - the act of probate.
I don't know what happened to the original will. I do know that in the 17th century, executors were required to present probate accounts to the court, to demonstrate how they had distributed the estate of the deceased - so perhaps the executors held onto the original while they were administering the estate. Copies were presumably made because they were required for different purposes.
I'm no expert in this field, but there are books available which can give you further information if you want to find out more.
The National Archives' catalogue entry for original wills in PROB 10 states:
"The majority of the documents in this series are original wills signed and sealed by testators and attested by witnesses.
Includes original promulgated sentences which were bundled with original wills until the mid seventeenth century.
The rate of survival of original wills is uncertain before the early seventeenth century and particularly poor before the mid sixteenth century.
Many of the pre 1600 documents are copies, the original wills having been returned to the executors.
Late eighteenth and nineteenth century wills are often endorsed with the date of the testator's death.
Some original wills are written on printed forms which testators filled in with simple instructions as to the disposal of their estates.
Includes a few items which may once have been kept in the bundles in PROB 31 and PROB 33.
Arrangement The vast majority of the wills and sentences in PROB 10 were registered in the will registers in PROB 11 , and for most purposes there is no advantage in examining an original will or sentence in this series, in addition to a registered copy in PROB 11 . However in a very few cases, wills were written in other documents such as diaries and account books, and some of these documents survive in the series.
Furthermore original wills may contain more personal information such as signatures. Many of them also contain seals.
Arranged in chronological bundles order"