I am not sure of the controversy on whether or not he must use his fingers to shamble. I just don't see it as particularly relevant.
Whether or not Law must use his fingers, there are other good explanations for him not shambling. First, two obvious ones:
(1) Strings - Dofla has immobilized people with invisible strings before. We do not know exactly how Law's power works, whether it creates extra dimensional space in which to travel, or if it involves moving through regular space in a strange way. If the latter, it is possible that Dofla's strings could restrict that movement - but I think we may have counterexamples, and even if not I don't like this explanation; it requires too much speculation on the exact nature of ope ope no mi transportation - it may be impossible for us, the readers, to get the metapysics correct here.
(2) Stamina. I don't know if "stamina" is an important resource for Law, or should be understood in a special way regarding Law; if I recall, we only heard about stamina on the bridge to green bit, and it really may just be that Law wanted to be fresh for his current (perhaps finished) fight with Dofla. At any time, we could say he was too exhausted. This isn't especially satisfying, so I'd rather endorse another explanation, but first let's tie in the fingers just to get another potential solution: Perhaps Law must be very focused to use his powers without finger gestures, and perhaps his stamina running low disrupted this focus. Furthermore, relatedly, he was certainly not focused at the time of the incident in question regardless of stamina - Dofla was getting in his head.
Even more simply, focus yields an explanation that does not depend on Law's fingers or stamina or anything…
(3) Dofla was getting in Law's head at the time of the incident. He is fighting Law physically and psychologically. Anyone who thinks that this is an ineffective strategy has probably never been in a fight, certainly not with someone who tries to get in your head. If someone gets in your head in a fight, big mistakes can be made. This explanation strikes me as so incredibly obvious that I just don't understand the big deal being made of Dofla grabbing his hands or whatever, except insofar as that might also be a symbol of domination reinforcing Doflamingo's psychological attacks on Law.
So, I endorse this: Dofla was fucking with Law's head, and Law simply fucked up. He wasn't thinking clearly, was filled with anger and fear (as is typical for his character - he is entirely motivated by his hatred of Doflamingo), and lost to Doflamingo's mind games. He has even stated Doffy's ability to get in your head before - and we have seen that it is one of Doffy's specialties - and warned Luffy to be careful to avoid that.
If we want to get extravagant, we could even say it was just enough to disrupt his focus, requiring him to make finger gestures to shamble, and Doffy took that option away by grasping his hands. I don't think we need to get this extravagant, since I think it is far simpler than that, but if you think that (i) the fingers thing, (ii) Law's lack of focus/Doffy's psychological attack, or (iii) Doffy immobilizing Law's hands are not good explanations each on their own, this would represent a more coherent theory that can bring in all these factors, and without appealing to the arbitrary "he was just out of stamina at that exact moment and not a second sooner when he was flinging towers" theory.
Also, just to note, if you like the more extravagant theory:
It seems to me that whenever we see Law shamble without hand gestures, he is relatively calm, either just going somewhere (Teleporting with Chopper), or - at most - just confidently being a smartass (the sanji case was more of a "Hey, fuck you buddy!" than either a "OH SHIT DOFFY MUST KILL DOFFY FOR CORAZON!!!" or even a self-doubt moment). He is never afraid or overcome by anger or anything like that when he shambles without using gestures.