Hi Yev,
This is probably a bit "late in the day" as you have probably now had you items appraised and valued. But for the benefit of other readers........
To answer your first question:
Choose your appraiser/valuer by reputation. If you are looking for someone who is specifically a gemmologist, look for the the suffix FGA and/or DGA after the Appraiser's name. These denote that the individual has qualifications with the Gemmological Association of Great Britain. Also look for the suffix AJA as this denotes that they are a Certified Appraiser with the Association of Jewellery Appraisers and by default need to be an FGA.
Check out valuers who are Certified Appraisers and/or Registered Valuers. Links to their sites below.
www.ajappraisers.org
www.jewellers-online.org
To answer your second question:
Many retailers do send their jewellery appraisal work to independent jewellery appraisers but if this bothered you, you could always approach the independent appraiser direct, many of whom, like myself, offer a service where you can attend their laboratory/office and watch the work being carried out or many offer a service whereby they would attend your home and carry out the work there. Either way, the items do not leave your control and as such would remove your worries about loss.
Also bear in mind that a Registered Valuer or Certified Appraiser is duty bound to provide a secure service backed up by a comprehensive specialist insurance.
In closing I would also state that in the thirty something years that I have been in the trade, losses along the lines of your concerns, are extremely rare.
Hope this helps
Kind regards
Adrian Smith