Recommendation, ignore speedster, you all already know how he is and he don't actually know about the series.
Regarding the resistances, having a bit mana pool in verse just give resistances to magic in general, to explain there is this analogy: if a bucket full of poison is throw in a small puddle the puddle will be contaminated, however if said bucket is instead throw in the ocean the ocean will be fine because is a body of mass so massive that such small thing is insignificant to it (is essentially speaking the same reasoning of why people with large sizes get resistance to various things). Now, in verse Makoto mana pool is so absurdly big that the mana pool of several gods would be needed together to compare to it, so that's why his resistances are above the rest of the verse.
Regarding Saikai/the field, think of it like an area were he change the laws of things, so for example if he change the laws to "magic is not allowed" then no one inside the area would be able to use magic, including him. He can freely manipulate its size and use it with different purposes, like buff himself (specifically just him), detect things, increase recovery speed, heal, block actions/null things, etc., There really is not limit to its versatility, reason to why for example in this case against Doom he could use it to restrict the use of technology or even flight if he so wanted.