"It would be less effective against the 'auto-forging' type of HEAT warhead, where the liner is the projectile rather than the gas jet"
Second things first, slat armor is generally ineffective against Explosively Formed Projectiles (self-forging projectiles) because their effective detonation range is in the 10-50 meter range, so there's no need for them to get close enough to contact the bar armor. Because the space between the bars is generally much larger than the the diameter of the slug projected by the EFP, the slug will usually miss the slats, making them irrelevant. If the slug does stike a slat, cutting it apart will be somewhat effective (reducing the mass of the projectile), but the most severe effect occurs if the slat casuses the slug to tumble. That puts less mass per surface area behind the strike, which vastly reduces its penetration.
The penetrator in a HEAT warhead is the copper liner, not any "gas jet." Copper is used for its ductility and moderate density. Gold is a much more effective liner because it is even more ductile and vastly more dense than copper, but obviously it's far too expensive to be practical.
While the gas jet has a small ability to damage armor, it could only penetrate the thinnest armor, and is really only effective against common steel (or common aluminum). Contrary to rumor, there is no "plasma" jet for the simple reason that the explosives in a HEAT warhead don't have enough energy to convert the liner to plasma (and certainly not the gasses of the explosion product) through any combination of temperature and pressure. The combustion temperature is far, far too low.
"Depending upon where it hits, and the angle, it could even make the cone of molten metal totally miss the target, or spray across it at an undesirable angle."
The jet is solid, not molten. This was proved experimentally by section HEAT warheads into disks, clamping them very, very tightly, and the detonating them to fire into water tanks. If the copper was molten, the sections of the the liner would have welded/melted together. The detonations were normal and the jets formed normally, but they simply fell to the bottom of the water tank, sectioned corresponding exactly to the pre-detonation cuts. You can also do some simple calculations of the energy outputs of the explosive, account for 50% loss to the part of the explosion directed outward (equal and opposite reaction), the amount of energy needednyo accelerate the liner to its maximum velocity, and approximation of percentage of the explosive energy going onto the "squeeze," and the energy needed to melt that much copper. The math just doesn't work out!
Please note that the term "jet" was an unfortunate xhoice and is very misleading. It implies that the copper liner is liquid or even gaseous, but it isn't. "Jet" was used because its formation occurs under fluid dynamics: the pressure is so high that the ductile copper flows like a liquid, so you can use fluid dynamics equations to model and predict jet formation with different cone geometries.
The slats can't cause the jet change direction because it simply slices the warhead lengthwise, preventing simultaneous detonation of all of the explosive in the warhead and changing the geometry and dynamics of the detonation, which prevents the jet from forming.
Jet formation depends on symmetrical and carefully timed detonation of the charge to drive the liner forward while also compressing it from the cylinder in. This inverts the liner, with the tip of the cone (at the rear of the warhead) forming the tip of the jet. If the warhead is sliced anywhere along its length and separated slightly, the pressure becomes completely unbalanced and the line escapes through the separations on either side. The result is completely ineffective.
"Also makes it/them detonate from too far away from optimal detonation distance, which further degrades the effectiveness of the weapons, if they go off at all." Fuses on HEAT warheads are very small, so the chance of hitting a bar is tiny. Not a significant part of the defense. You do this with solid spaced armor, if you this is what you're trying for.
"Isn't the main purpose to avoid the contact fuse hitting anohysica+"
No. The bar armor will not bring a HEAT warhead or missile to a stop. The expectation is that the fuse will still trigger, but that the explosive will not form a jet. Some fuses will detonate early due to the shock of the projectile hitting the slats, but that's unreliable.
*That isn't guaranteed, obviously, but then the other effects like stand off, worse angle etc etc are still hopefully going to increase your chance of survival."
Again, bar armor works by disrupting jet formation.