@No Way -
In my experience (where experience = 15 years / 6 Exchange generations / 3,2 million mailboxes / 0 failed migrations) 95% of the problems that occur during migrations are caused by very basic configuration oversights, not by the product itself. The problem regularly begins with putting the CD and running into migration without absolutely any checking of the existing AD and Exchange environments. A number of very very basic details get missed, for example: DNS suffix not entered on the machine, wrong binding order of network adapters, power management activated on network cards, antivirus exceptions not configured, execution of updates without RunAs and while antivirus is running in the background, reference to non-existing DNS or WINS servers, polluted WINS databases where 5 machines refer to same IP, failing AD replication....
If we specifically talk about your case it is a classic and I and the root cause is most certainly that antivirus on that machine does not have antivirus exclusions entered which brakes the virtual directory on a regular basis.
If the people that you have engaged are not doing the job it might be time to try with someone else. We can offer you is to perform such standard health check on your environment and try to sort out the problems you've been experiencing. I run my own Exchange specialized company, Exchangemaster GmbH in Zurich Switzerland (Central European Time Zone) and we re a Microsoft Silver Partner with multiple Microsoft Exchange MVPs on staff. In case your company has Software assurance/Enterprise Agreement and you have those MS consultancy vouchers at hand, you can use those to pay for our services. Remote access via VPN should be sufficient and we can perform that regardless of where you are in the world. My e-mail is dejan.foro@exchangemaster.net