One of the main weakness of Skywrath Mage is the almost total lack of armor, so Mekansm and Ring of Basilius are the best items to buy in order to partially solve this problem (considering they are cheap, in comparison to Shiva's Guard and so).
Even if you're able to position yourself correctly, there are still many ways in which you can be involved (example: Blink).
I don't like that much your argumentation for buying Null Talisman. In the guide I repeated many times I would rather skill Arcane Bolt later and, also, I want to analyze this suggestion with a concrete hypothesis: at level 5 (with level 3 arcane bolt) you deal the misery of further 14 damage and, above all, 2.6 less armor points.
You could answer to my first assertion about the lack of armor saying something like: «Tranquil Boots provide you a +3 armor bonus». They aren't a bad pair of boots but, the fact is, Arcane Boots totally overhang them, due to the other well known problem of Dragonus: mana.
Of course Guinsoo's Scythe of Vyse is better than Rod of Atos, but it's very hard to farm and so the latter seems the best choice, since it has various utilities.