They could have replaced the "ri" in skyrim to a "n" or a "h" making it Skynm or Skyhm. This is what they could do if they lose the trademark. Quick thinking people could trademark Summerset isle and then wait for BGS to pay them for it.
Changing a letter in Skyrim is ridiculous. No one ever does this to their own brand; only those who intentionally try to knock off a brand.
They have not obtained any trademarks to lose. Until they prove "Skyrim" is being used in commerce, the so-called "trademarks" are merely applications for trademarks but not the actual things themselves. One quick way to know this is to notice that none of the "Skyrim" records pertaining to Zenimax Media have a registration number, which definitively indicates that they are not yet actual trademarks.
As for the Summerset Isles idea, anyone who would apply for trademarking "Summerset Isles" would be faced with two problems. First, they would have to prove its use in commerce within 36 months (which includes all extensions) or it would be lost. Another, more serious issue, is that it would be a blatant attempt at squatting and would risk litigation.