So what's close? 2 laps? 3 laps? 4 laps? 5 laps? If it stuck 4 or 5 laps before his scheduled stop it still would have thrown off his pit schedule. If he waited longer and didn't have to the chance to use his DRS he wouldn't be allowed to engage it before his stop. Again you are just assuming the opportunity would have been there.
You're assuming the DRS was a necessity in order for Ferrari to remain competitive--Alonso's result shows that it wasn't, and if they had instructed Alonso to keep the DRS closed until around their next scheduled stop, they would've realised this early on in the race. They really didn't need to test it again at all, and personally I would've only considered a test of the system again if Alonso quickly began losing time on the leaders--then it would have been worth taking the risk because they would have had nothing to lose.
Testing the DRS on a lap mid-stint may have compromised his pit schedule, but again--would four or five extra laps on his last set of tyres really cost him as much time as an additional pit-stop did? Regardless of whether the opportunity would have been there or not, I stand by my original opinion: it was an unnecessary risk, and they should have waited until they were close enough to their expected pit window to test it, in order to cover themselves in case it wasn't working properly.
In essence, Ferrari made a decision on the value of DRS in the race almost immediately after Alonso's first stop. I think it's logical to assume they wouldn't have had Alonso test it so soon after his first stop if they thought it was going to be unimportant. Their mistake lies in the fact that they didn't wait to see how competitive Alonso was without DRS
first before making a decision on how useful it would have been for their strategy and the race. Basically, they made a decision on how effective Alonso's car would be without DRS going on, what, a lap and a half of data? That's really where my disbelief over this whole situation lies--it strikes me as being very unlike Ferrari to take such a risk in a race that looked very early on like being Vettel's. They needed to be on damage control, and they weren't.