There's no need to house rule this — you just need to start enforcing its restrictions. Allow me to draw your attention to its area of effect:
4 square″ + 1″ square/level
(Aside, this is taken from PHB page 95, since the Illusionist version of the spell has different range and area of effect.)
These are table inches, which we could convert to feet/yards, but the following will be simpler if we just compare table inches directly with fireball's numbers.
A fireball has a radius of 2″. Let us ask then, “what level would an illusionist have to be to cast a phantasmal force large enough to contain a 2″ radius sphere?” Let's make a first-order approximation using the minimum number of 1″ squares necessary to create an area in which the image of the sphere could appear. The area of effect of fireball is three-dimensional, but the area of effect of phantasmal force is two-dimensional, which presents an odd issue. However, phantasmal force is described as being able to create images “within the boundaries”, so let's be as generous as the spell seems to be intended, and say that the vertical space it can create the illusion within is “good enough” for our purposes and we just need the footprint of the illusory fireball to fit inside the phantasmal force's area of effect.
We'll assume as a first-order approximation that each 1 square″ of area of effect can't be divided, so they have to be tiled as whole square table inches. Inscribing a 4-unit sphere's footprint (a circle) inside a pixelized shape requires a 4×4 square of squares, or 16 tiled squares.
So, as a first approximation, an illusionist needs to have 16 square table-inches of area of effect to play with to be able to make a phantasm of a full fireball. That requires being level 12 to pull off.
But maybe you think tiled squares aren't generous enough. Let's allow the table-inch squares of area of effect of the phantasmal force to be subdivided infinitesimally so that the area can be a cylinder that just barely contains the footprint of an illusory fireball and see where that gets us. (Some DMs might reject this move since the area of effect is given in squares and they may require a straight-edged area of effect, but some others might allow it with the reasoning that it's fine as long as the final area matches regardless of shape.)
A 2-unit-radius sphere has an area through its widest point of just over 12½ units, rounding up to 13 square″ required. So even being super-generous, it would still take an illusionist of level 9 to use phantasmal force to create an illusory fireball.
Clearly, creating illusions of fireballs with phantasmal force isn't something that your players should be doing at first level, or even 5th level when their magic-user friends are starting to throw real fireballs.
Conclusion: You don't need to house rule, just give the players a reality check
So, there you have it: you don't need to house rule this at all, you just need to pay proper attention to the relatively small area of effect, and break the news to your players that you've been accidentally letting them create a 9th-level illusionist's phantasmal forces before they were actually 9th level, and won't be doing that any longer.
If they are 9th- or 12th-level, then this isn't overpowered at all, because they have access to equally (or more) powerful effects already and cleverly using their resources like this should be letting them paste low-HD enemies by the truckload. So again, there simply isn't a problem if the area of effect is enforced.
Addendum: Don't overlook the audio limitation either
The above is all completely ignoring the impact of the stipulation that a phantasmal force is silent. Is a fireball silent? Not at all. An illusory fireball with no sound effects is not very convincing, and unlikely to fool even stupid opponents, even stupid opponents who have already been hit by a real fireball. In fact, having just experience a real one with all the sound and fury of real fire, they'd be more likely to notice that a silent one is not quite right.
There is no RAW answer. It must be decided by the DM.
Firstly, this ambiguity is likely an intentional, direct consequence of 5e's design ethos of "rulings over rules" and the prominent role of the DM as the person that must make those rulings in the case where the rules are silent. Illusions and illusion magic are one area in which there is essential no general rules guidance, and thus falls squarely into the domain of DM-rule.
Jeremy Crawford said as much when asked about this exact question on Twitter:
Q: Can a creature under the effects of Phantasmal Force be restrained by it, i.e chains wrapping around a creature?
A: Phantasmal force details the few game effects that it can reliably create. A DM is free to allow additional effects.
Because illusions are a complicated issue, I will outline two of the major DM approaches to this issue. Since they take opposite stances to each other, any DM reading this will have to figure out which way makes the most sense, seems the most fun. and/or which way aligns better with how they've already been handling it.
The core of the disagreement is whether the DM rules that an illusion fools the mind so thoroughly that the body will react physically to it or not.
Interpretation 1: No, Phantasmal Force cannot chain someone and force them prone
In this argument, the spell looks and feels like a chain, but the chains (not being real chains) would not be able to restrain the creature since they would offer no resistance when the creature tries to struggle out of them.
The target rationalizes any illogical outcomes from interacting with the phantasm
Because of the above effect from the spell, the mind of the creature would try to rationalize this in some way (the chains broke, the chains loosened temporarily, etc.) but they would still believe the illusion to be true.
In this way the creature would not gain the prone condition because their movement is not actually inhibited in any way even though they believe the illusion is real.
See this answer for a more thorough take on this interpretation.
Interpretation 2: Yes, Phantasmal Force can chain someone and force them prone
In this interpretation, the chains really do restrain the creature because the creature treats the chains as real chains, even though they are not.
While a target is affected by the spell, the target treats the phantasm as if it were real.
The chains also sound and feel real to the target in every way.
An affected target is so convinced of the phantasm’s reality that it can even take damage from the illusion.
So, in this case, when the creature tries to escape from its restraints, its mind, thoroughly convinced that the chains are real, will act under that assumption. Thus, the creature's limbs will be bound. And this creates the case for the fact that the creature is able to gain the restrained condition from this spell.
See this answer for a more thorough take on this interpretation.
Decide but stay consistent
So I have provided very brief overviews of two interpretations of the spell's effect. There are certainly more of them out there. As a DM, the key is to decide on one and apply it fairly, transparently, and consistently. Not only across this spell but other similar ones.
Best Answer
Truesight does not interact with phantasmal force
Truesight lets you
but with phantasmal force
A visual illusion is a trick of light, but what this spell creates is a trick of the mind. Having better vision will not help with something that is implanted directly into your head. So the statement of your 3rd case is right, but for slightly different reasons.
Caveat: This is an interpretation of the rules. Without a setting behind it that explains how illusions work precisely, other interpretations might also be rules-legal.