The reactivity of halogens :- The above order in case of halogens is due to the fact that the size of fluorine is small which makes a strong bond with carbon of alkyl group, thus makes it difficult to break.
But on the other hand in case of Iodine the bond between Iodine and Carbon is quite weak and its bond dissociation enthalpy is also quite low. Thus this bond can break easily and it makes the alkyl iodides more reactive.
Reactivity order of alkyl groups :-
The dehydrohalogenation reaction involves the formation of Carbocation. Out of primary , secondary and tertiary Carbocation, the tertiary Carbocation is most stable because it’s charge is compensated by the three alkyl groups attached to it. Thus the 3⁰ (tertiary) carbocation is highly reactive.
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