Thank you for the additional information.
The issues will include but are not limited to:
I question how a Solicitor can admit to witnessing an unsigned will? as he should have witnessed the will AFTER your Father had signed it.
This same Solicitor is going to be faced with a conflict of interest here, he is acting for the Executor - yet also acted for your Father in the preparation of the second will, which he has potentially witnessed before your Fathers signature was on the document, he is in trouble on that point alone.
Setting that issue aside, he is potentially your witness as well to confirm that the unsigned will was your Fathers last will, albeit unsigned - yet at the same time he has to act on behalf of the Executor, he does not make for a credible witness.
This is just a very small summary, I am sure there is more if all parties are interviewed to answer the fundamental question - which will IS the valid will?
Its not my place to draw any conclusions here as there is clearly going to be a dispute of some description, as you are the children you have a valid right to contest the will if you have not been adequately provided for - you do need to take your independant advice and bear in mind that the costs of contesting the will may well dilute the estate as a whole, hence the importance of both parties settling on a compromise to preserve the assets for distribution.
I trust this clarifies the question for your acceptance, my original answer on costs here could be narrowed down to under $5000 with agreement, up to $50,000 without and the time frame remains unchanged depending on whether it is mediated or determined in the courts.
I wish you the best in any event