You only get extra magic resist when facing an AP heavy team, and even then you can rip through those champs because they tend to be a little weaker on the hp. There really is no reason to boost your AP as Rammus when you are tanking, it takes away from what you are meant to be doing.

The main reason why getting armour is generally better is for the AD, and combined with tremors, taking out turrets solo or with a partner is easy. After a fight, if Rammus is still alive, he needs to heal anyway, so increasing mana is again, pointless. I never need more than what I have, and never had a need to buy items that increase mana.

Treads are not needed at all. Rammus needs to move, slowing him down takes away from what he is the best at. If you need the extra magic resist, go big or go home. Simply buy a 2nd Force of Nature. Not only will that ramp up your hp regen by another 40, you get another 76 magic resist. But like I said, you don't need that much magic resist unless you are fighting a heavy AP team.

And add some alternative/situational items, for example Abyssal scepter Gives you M.Resist, AP (Boosts your ult) and reduces your nerby enemies M.Resist (Great way to support your ap allies)

Hmmm you should get Mercury's Treads in my opinion.
And yes you get ad from your armor, but you are a tank so try getting more M.Resist, For example changing randuin's for a Banshee Veil this gives you more HP/Mana and you are harder to cc becouse of its passive, great for a tank, and you wont get binded when chasing with Powerball

And try adding some images to your build, its so sad and boring read that big mass of text.

You see other Rammus builds using other boots. And you have to ask yourself, why?

His cooldowns are short enough, so that rules out buying Ionian Boots of Lucidity.

The objective of Rammus is to get hit so damage is returned to the attacker, so that rules out Ninja Tabi. His armour gets plenty high anyway, so the +25 is not necessary.

Magic Resist can be useful, but Mercury's Treads only gives +25, and for most practical purposes, you will need more, and from something that gives more bonuses too.

Boots of Swiftness might seem ideal, because you can move around at a faster speed than normal at all times. Faster than almost every other boot too. But that is it.

Berserker's Greaves, uh no? You're not the attacker, you don't need attack speed.

Boots of Mobility is the bread and butter Rammus needs. For him, it is not about how fast you can move around during combat, it is about how fast Rammus can get into and out of combat. When running away the extra speed is great when it kicks in, especially if Powerball is not available at that time. Getting into combat to save your allies after reviving or healing up is crucial at times. And the only way to do that effectively is with the best speed possible. These boots + ghost + powerball.