If under FL law, they could not arrest or detain Zimmerman, then they likely could not force him to give a blood sample.
I'm not so sure the police interpreted the law correctly. For one thing, the law does not appear to authorize
pursuit except in cases where a "forcible felony" has been directly witnessed by the would-be stander-of-ground. There is no dispute that Zimmerman did not witness any crime committed by Martin. The fact that Zimmerman had only a hunch to go on has a direct bearing on what sort of tone it was reasonable for him to take upon coming face to face with Martin. And reasonable is the name of the game, here.
Also, one could reasonably argue that, absent any evidence that Martin was doing anything unlawful, it was unreasonable for Zimmerman to be so abrasive in approaching Martin. A polite and conciliatory explanation of what Zimmerman was doing and why he was following Martin ("there have been break-ins in the neighborhood, and I don't recognize you, so I just wanted to ask you what's up. I hope you'll understand that I'm just looking out for my neighbors") would likely have averted any need for physical confrontation. Martin would likely have been offended by that (reasonably so), but it probably wouldn't have triggered a fight or flight response in Martin. He would probably have been annoyed, but he probably wouldn't have felt threatened, since he had a perfectly reasonable explanation for being in the neighborhood.
Given that, the police could reasonably have determined that Zimmerman's actions were unreasonable under the circumstances, and
that is enough for them to make an arrest, because it would make Zimmerman's use of deadly force unlawful per the SYG statute.
Now, perhaps the police couldn't have had reason to doubt Zimmerman's reasonableness until after they spoke to Trayvon's girlfriend (which required getting phone records), in which case the failure to arrest at the scene was reasonable. But now, it seems pretty clear to me that Zimmerman was far more abrasive and confrontational than was reasonable under the circumstances.
A reasonable person would have considered that there was a good chance nothing nefarious was afoot, and that it is reasonably foreseeable that even (perhaps especially) an innocent person would be offended and defensive at being suspected when they were innocent of any wrongdoing. From the 9-1-1 call, it appears that Zimmerman did not merely suspect that Martin
might be up to no good, but that he in fact
assumed, without evidence, that Martin was in fact up to no good. And that assumption appears to be the
sine qua non of this tragedy. It isn't as if Zimmerman was forced into a situation where he had no time to consider this sort of thing.
If I were the special prosecutor, I would feel as if I had a strong enough case to proceed. The 47 (or whatever) 9-1-1 call for trivial reasons may not be admissible in court, but they are nonetheless probative of Zimmerman's propensity to be unreasonably over-zealous. It is completely appropriate for a prosecutor to consider such evidence in deciding whether to pursue charges, as long as the prosecutor also believes there is enough
admissible evidence for a reasonable jury to return a guilty verdict.