Peptide oxidation bias during sample preparation for LC-MS/MS
I know this was posted a long time ago and I'm not sure if you have found your answer but I am an author on two of the publications you  andcite can offer some insight.
1.) Methionine oxidation, unlike cysteine oxidation, is irreversible without the action of a dedicated enzyme or catalyst. Neither TCEP nor DTT is able to reduce methionine sulfoxide. The only reducing agent that I know of that is strong enough to reduce methionine sulfoxide is MMA.
2.) The in-vitro oxidation of methionines during sample preparation is a well documented issue in LC-MS/MS analysis and it is known that the majority of in-vitro oxidation occurs during peptide digest as well as during electrospray ionization. The strength of the effect is highly variable and dependent on how long you digest proteins. For a typical overnight digest we observe effects that range from 0%-400%, depending on the peptide.
3.) The use of 18O labelled blocking reagents is the only currently available method that can prevent the accumulation of in-vitro oxidation. I'm the first author on the publication you cite and would be happy to send you a copy if you email me (jbetting@ur.rochester.edu) I have also been working on improved methods using the same general strategy and would be happy to share those with you after publication