Monk and bonuses to monk level
If you are a monk, a monk’s belt explicitly makes you count as a monk five levels higher. There is no question as to whether these “accumulate” (usually called “stacking” in 3.5e parlance), since the monk’s belt explicitly accounts for what happens if you are already a monk.
And gauntlets of the talon also explicitly note what happens if you are a monk, and/or have a monk’s belt—they also definitely stack.
For example, a Medium 2nd-level monk with monk’s belt and gauntlets of the talon counts as a 12th-level monk, and so has a 2d6-damage unarmed strike. It’s still a Medium weapon, it’s just a Medium weapon that deals 2d6 damage, like a Medium greatsword would.
Note that a monk’s unarmed strike damage explicitly does not increase past 20th level’s 2d10 damage. Neither of these items, nor more monk levels or other levels that stack with monk levels, can increase this.
Fist of the forest
On to fist of the forest. Fist of the forest is weird. It refers to whether you “already” have the unarmed strike damage listed, which is one of those words that often gets us in trouble in D&D 3.5e. The general consensus is that “already,” like “normally,” means “without this effect we’re talking about now.” This result surprises many people, but generally speaking, ordering shouldn’t matter to a character, and when it does, the recommended rule of thumb is that things can be ordered any way you like. Thus, you can adjudicate the fist of the forest’s unarmed strike damage increases after adjudicating the monk’s changes, including monk’s belt and gauntlets of the talon, even if you didn’t “already” have those at the time you took the fist of the forest level. This doesn’t sit well with some people, but all I can say in the space of this answer is, trying to do it any other way is near impossible to keep consistent without causing far weirder effects.
Our monk from the previous example had unarmed strikes that dealt 2d6 damage. If they now take a level of fist of the forest, it says that if you “already” had unarmed strikes that deal the damage listed, you instead increase the damage one step, per the monk unarmed strike table. That’s equivalent to 4 monk levels, by the way. It’s unclear if fist of the forest means one step ever, for taking the class, or one step each time fist of the forest would have improved one’s damage. One step would be 2d8; two would be 2d10. Anyway, we’ll assume a 1st-level fist of the forest here, just to keep things clear—so 2d8 damage. Again, still a Medium weapon.
Again, since we are supposed to increase a monk’s unarmed strike damage by one step on the monk’s table of unarmed strike damage, this ability probably cannot increase the damage beyond 2d10.
So if that monk also applied morphic weapons to their unarmed strike,¹ you would look up 2d8 in the Medium column of the Smaller and Larger Weapon Damage Table, and then find the next value to the right for Large. Problem is, there is no entry of 2d8 in the Medium column, because no weapon normally deals that much damage at Medium size. The usual extrapolation here is to shift everything over by a column: from the 2d8 entry in the Large column, look up the damage for Huge. So 3d8. Of course, the monk/fist of the forest/warshaper has also devoted at least 4 class levels (and that ignores how they actually qualified for fist of the forest and warshaper), as well as 32,000 gp and either a 4th-level spell slot or a feat.
Bear warrior and animal growth
Now for bear warrior and animal growth. First, natural weapons and unarmed strikes are separate. None of the benefits of monk apply to the bear form’s claws or bite—those are claws and a bite, not an unarmed strike. A bear can (probably, nothing says it can’t) also perform an unarmed strike à la a humanoid, but it sees no benefits related to claws or bites or other natural weapons. As a black bear (per the single level of bear warrior indicated in the question), the monk/bear warrior/warshaper is still a Medium creature, all of the above applies. Then we add animal growth, so that we are now a Large bear. Since we are in the “form” of a Large bear, between bear form and animal growth, morphic weapons says² we can make our unarmed strike deal damage as if one size larger still—Huge. So we go back and look up our 2d8 damage in Huge (because Medium doesn’t have any), and go to the right two columns for Gargantuan (because we want Huge relative to Medium but we have to start from Large)—4d8.
Finally, imagine we had 5 levels of bear warrior, and took the form of a brown bear—a Large bear. Animal growth brings us to Huge, and morphic weapons increases our damage from that of a Huge creature to that of a Gargantuan creature: 6d8. Pretty hefty weapon, really, averages 27 damage. But consider that this required 9 levels, 32,000 gp, a 4th-level spell slot or a feat, another 4th-or-5th-level spell, and can only be done a few times per day. We’re a minimum of 13th level (since bear warrior requires BAB +7 and we have at least 3 levels of ¾ BAB)—a rogue at that level gets +7d6 damage on a sneak attack (average 24.5), and that’s on top of whatever the weapon itself is doing. And a rogue isn’t really a stellar damage-dealer. And we haven’t given the rogue any feats or any kind of magic gear or spells.
Which isn’t to say it’s a bad idea: fist of the forest, bear warrior, and warshaper have plenty of other things also going for them. Far fewer targets will be immune to your damage. You’ll likely be a lot more accurate, and definitely a lot more durable, than a rogue. It’s just that you’re not setting any damage records.
We’re ignoring the poor wording on morphic weapons that suggests you might be able to keep applying it over and over on the same natural weapon, because then the damage just becomes arbitrarily large, or we get into arguments about whether or not the rules support extrapolating larger damage sizes past Colossal.
Morphic weapons explicitly says that it increases our natural weapon’s damage relative to the weapon that “form” would have otherwise. This is unusual—often the rules will refer to what “you” would “normally” have. The general consensus is that “normally” means “ignoring this thing we are talking about right now,” but it’s not super clear, and there are those who disagree. Morphic weapons just happens to be one of the few cases where we don’t have to rely on that, so it’s a lot clearer that this works.