This would all be a massive overreaction.
I get why they would deal Betts, but the idea should be bring him back after one year, which means they need a tea to bring him back to. If he is set on testing the market, Boston has a strong history of making competitive offers to free agents. Cincinnati is one possibility, but I wouldn't rule out the Mets. And I think the most likely targets are San Diego (where the CEO has promised "heads will roll" if the team has another embarrassing season, and the Braves. Both alo have much, much stronger farms than Cincy, and Luis Patino is a better trade target than Hunter Greene. (The Padres also have a massive 40-man roster crunch, and the deal could be expanded to include some of the prospects needing protection for some of the Sox who do not.)
I get dealing Walden, but honestly, a 30yo career journeyman even with a low salary and a lot of control isn't likely to bring back much else besides another career journeyman. If the Sox dealt him for, say, Brian Goodwin, that's about the limit of his appeal.
I do agree it is not likely JBJ is back. I think it is more likely Rusney Castillo is in the Sox OF next year, but I wouldn't bank on that, either.
JDM isn't likel;y to opt out, and if he does, it gives the Sox about $78mill to spend before reaching the tax limit. With very little else is the way of appealing hitters on the FA market, the only thing more surprising than Martinez opting out would be the Red Sox not bidding heavily to bring him back anyway, and probably at a lower AAV (although with another year or two tacked on). Also, the buzz in Chicago is that Abreu and the White Sox have mutual interest in getting something done.
I don't see Barnes as a "sell low" and I don't think GMs view players that way anymore, especially not players under 30. Barnes is a 1-1.5 fWAR relief pitcher with 2 years of control left, and there is a lot of value in that. He ranks 22nd in MLB in fWAR over the past 3 years among all relief pitchers, and third among all relief pitchers with 5 or fewer saves. So as non-closers go, only Chad Green and Yusmeiro Petit (who is always criminally undervalued) have been better. If the Sox deal Betts, Barnes salary, which might be $2-3mill, is hardy an obstacle and he can easily be retained if no one meets an asking price.
The Sox are NOT going into some stupid full scale rebuild. They cannot afford to, and have too much money on the books for the next few seasons. They will try to be competitive next season, despite what Yankee fans want...