There are other ways of doing things, but this is certainly one of the recognised ways. It should not be too costly since all you will be seeking is proof of the Will in solemn form at law. I would get your solicitor to instruct Counsel and see what views Counsel has.
In terms of the burden of proof, if someone disputes a Will on the grounds of testamentary capacity the person propounding the Will (this means proving - so in this case, you) must prove that the deceased had testamentary capacity. HOWEVER in terms of undue influence (which is what you mean by coercion), the person alleging undue influence must prove it. It is an extremely high hurdle as well.