Why we need halogens of different periods in wurtz-fittig reaction? + which is the correct mechanism?

For the mechanism of wurtz-fittig reaction I got two different ones on the net.
1. From another [question](https://chemistry.stackexchange.com/questions/81983/does-wurtz-fittig-reaction-involves-sn1-or-sn2) on stackexchange: [![enter image description here](https://i.stack.imgur.com/1eIwx.png)](https://i.stack.imgur.com/1eIwx.png)


2. From tutorvista:[![enter image description here](https://i.stack.imgur.com/Jfs88.png)](https://i.stack.imgur.com/Jfs88.png)


They are not really the same. In the first one the alkyl group is acting as the nucleophile and as Sn reaction do not occur at sp2 hybridized orbitals, it goes through an SnAr reaction. But in the second one the aryl group is acting as a nucleophile. So the same reasoning does not apply here, because here Sn2 is happening at a primary sp3 hybridized carbon whereas the aryl group is just the nucleophile. So is the second mechanism wrong or I am missing some point? Neither of the mechanisms really show why we used two different halogens in the alkyl and aryl group (or maybe I can't understand), in fact the first mechanism is using Cl on the alkyl group whereas the second mechanism is using iodine (which is a far better leaving group). But the aryl group has bromine on both. Really confusing if I am to search for explanations myself. Please clear me up.






