|Mathwei ap Niall|
Monstrification giving Monstrous Physique 3 times a scenario when they can cast the spell themselves isn't a big deal. Likely they could have cast it twice themselves, so I doubt it'll be a huge change from where they were at.
Given you can't ready a charge, you also couldn't use contingent action on this. 125 gold a scroll does seem cheap however for this relatively powerful action economy cheat, although a 3min/level duration means you'd have to have some foresight. You could have a readied action to move to an enemy if it looks hostile, which would get you adjacent to one for your full round.
The ring doesn't state that it gives you the ability to use spell completion/trigger items as natural spell and wild speech do.
Don't forget you can recharge the Staff by burning a few extracts of enlarge person. By the time an alchemist can afford this item the will have 1st level slots and Boro Beads to burn to make sure they can always use this power whenever they want.
As for the ring, theres nothing stating you lose the ability to use spell trigger/completion items when polymorphed either. The only restrictions are
While in such a form, you cannot cast any spells that require material components (unless you have the Eschew Materials or Natural Spell feat), and can only cast spells with somatic or verbal components if the form you choose has the capability to make such movements or speak, such as a dragon.
Being able to speak, move and manipulate items is all that's called for to cast a spell no matter the form. A Sorceror with this ring can cast any spell they want with at most a +1 level adjustment (if the spell had a somatic component) by taking Still spell or using any of the free ways of getting a metamagic feat.
Finally, Contingent Action. Remember all with the rules for charge since the recipient of this spell is restricted to a standard action by the rules of the spell instead of a Full action they CAN charge while under the effects of this spell.
Remember, this Contingent action happens outside of the targets turn so they can only take the readied action the spell provides. And per the rules for charge since they can only take that standard action partial charging is legal here.
And since contingency effects are immediate they can easily occur before initiative is rolled. Every important fight will begin with the melee in melee range with the opponent before initiative is even rolled.