I’d like to throw my hat into the ring here, not for the sake of hoping this is a correct answer, but in an attempt to give, as much as possible, a complete picture.
TL;DR: Magi have a class feature they can choose to take over other class features that allow them to get wizard spells. By house-ruling that a magus can learn a wizard spell through independent research, the in-class feature becomes moot, opening their choices to other features more readily in a way the developers may have not intended. This could make an already powerful class unbalanced.
The wordings of the sections quoted are very specific, probably as intended by the game developers. Wizards would be defined as a class of their own that is separate from Magi, even if the flavor text for both classes indicate that they both undergo a fair amount of study to achieve arcane power. Lore wise, maybe wizards gave up practicing with the blade to do this type of research, while magi would give up further research by honing their skills with their weapons. Hence, I think it would be a bit of a stretch to say that magi could undertake independent research as wizards could (not saying it’s impossible, I know a lot of fit researchers, but maybe not to the extent that Magi are as fighters).
For Independent Research from Addings Spells to a Wizard’s Spellbook
A wizard can also research a spell independently, duplicating an existing spell or creating an entirely new one. The cost to research a new spell, and the time required, are left up to GM discretion, but it should probably take at least 1 week and cost at least 1,000 gp per level of the spell to be researched. This should also require a numbe of Spellcraft and Knowledge (arcana) checks.
(Bold is my emphasis)
Here the rule says wizard, which implies only the wizard is capable of duplicating an existing or creating an entirely new [spell].
For Knowledge Pool
At 7th level, when a magus prepares his magus spells, he can decide to expend 1 or more points from his arcane pool, up to his Intelligence bonus. For each point he expends, he can treat any one spell from the magus spell list as if it were in his spellbook and can prepare that spell as normal that day. If he does not cast spells prepared in this way before the next time he prepares spells, he loses those spells. He can also cast spells added in this way using his spell recall ability, but only until he prepares spells again.
(Bold is my emphasis)
The magus can treat any spell from the magus spell list as if it were in their spell book. It does not indicate that the spell is from another class’ spell list. Further, while it is a tentative “yes,” if I were particularly punctilious about the wording of the rules, a magus wouldn’t/shouldn’t be able to do this because there’s nowhere in the Magus rules that indicate they can do it. If they did, there would have been a rule in there that says “a magus can replace and copy spell books in the same way a wizard does,” as they had done with the witch’s familiar:
At 1st level, a witch forms a close bond with a familiar, a creature that teaches her magic and helps to guide her along her path. Familiars also aid a witch by granting her skill bonuses, additional spells, and help with some types of magic. This functions like the Wizard’s arcane bond class feature, except as noted in the Witch’s Familiar section.
I would stick to the wording in the rule, simply because the Magus already has a feature that allows them to take spells from the wizard spell list with a specific Magus Arcana: Spell Blending (Ex):
Benefit: When a magus selects this arcana, he must select one spell from the wizard spell list that is of a magus spell level he can cast. He adds this spell to his spellbook and list of magus spells known as a magus spell of its wizard spell level. He can instead select two spells to add in this way, but both must be at least one level lower than the highest-level magus spell he can cast.
Special: A magus can select this magus arcana more than once.
Magus Arcana is a feature that a magus can exploit at level 3 and every 3 levels after that. This makes it an expensive feature, so a magus must carefully select which magus arcana to have when the time comes.
So if a magus were to select Spell Blending, they are giving up a whole bunch of other powerful features in order to learn a spell from the wizard spell list.
Such a feature should be expensive with respect to losing out on other class feature options. In other words, a feature to get wizard spells wouldn’t/shouldn’t be something that a magus could access without losing out on another class feature a magus could choose from. Even if the magus would have to undergo several Spellcraft and Knowledge (Arcana) checks and spend a lot of money, there will come a point where passing these checks and spending that money will be easier at higher levels than lower levels (the only limit would be time, which may not be possible in very time-limited portions of campaigns). In that case, the magus wouldn’t lose out on another class feature.
Let’s take an Eldritch Archer magus archetype example. A magus arcana they have access to is Arcane Accuracy (Su). But another choice they could go with is Spell Blending (Ex), as the eldritch archer had their eye on Named Bullet, a spell that is only available on the Wizard’s spell list, for a few levels now. But with a house rule where they can take a wizard spell through independent research, the Eldritch archer doesn’t need to choose between Arcane Accuracy or Spell Blending, they can have the best of both worlds at the same level. In this case, the drawback is only less time and money. That’s where the imbalance is; the magus should have to give up some feature when taking up another feature.
So to answer your question, I would think that house-ruling a magus to research spells the same way a wizard can would be unbalanced.