yes thinking about it kinda difficult to finish a DTV until it has expired. You are correct the DTV can be converted to another type of visa at an Immigration Office but how easy that is for Chef's remains to be seen.
You can not work in Thailand on a DTV visa. However, there are plenty of foreign chef's working in Thailand so if you do get a job to legally work you would have to finish the DTV and then re-enter on another type of visa .
yes can be sold but the sale is only for the length of time remaining on the lease (max 30 years) there is no automatic right of renewal of the lease that is why it is very difficult to resell leasehold property.
if the application shows it is a genuine tourist visit and not a permanent move you will have no problems. Proof of sufficient funds and evidence your wife has a reason to return to Thailand helps.
Not too difficult if you follow the application process on UK Gov website, no need for an agent. Just involves lots of form uploading and your wife having an interview with VFS Global in Bangkok. Did it for my wife about 6 months ago whole process took about 2 weeks.