Homemade Tofu Firmness

You've probably seen fried, marinated tofu in those Thai dishes (my best guess without seeing a photo). The red colour would be from a short soak in either chili/garlic oil, or sweet chili sauce. Many asian markets sell tofu packaged in a marinade or breading for convenience.

1- Is this a valid approach and I should just add a lot more of my flavoring agents?

Yes, you can make additions to your curd prior to pressing/knitting just like with cheese. Adding bits of dried peppers ala pepper jack cheese sounds like a great idea. The main concern in this regard is to avoid adding so much adjuncts that the tofu curd fails to knit together during the pressing/draining phase and falls apart as you're handling it during subsequent cooking.

A secondary concern is that tofu generally weeps significantly more water than cheese curds. And since you've added your adjuncts already, part of the flavor of those adjuncts will run off as the tofu curd weeps. The only real suggestion I have for that is to use assertively flavored adjuncts.

2- Are there other flavoring agents that will present themselves more forcefully (and pleasantly) in tofu?

Soy beans have fat in them. The fat in the beans becomes emulsified in the milk when processing the beans into soy milk. The majority of that fat ends up in the soy curd when curdling the soy milk. The majority of the fat in the soy curd will remain in the curd during pressing/knitting into tofu blocks.

You can use this to your advantage. When grinding the soy beans into milk, you can fat based flavoring, some of which will end up in the soy milk, some of which will end up in the resulting curd, and thus into the tofu blocks. If your fat based flavoring is particularly intense, then it should be noticeable in your tofu blocks. A common type of intensely flavored (aromatic) fat based flavoring is essential oil. You can add essential oil of lemon or orange and that flavor will certainly carry through into the final tofu blocks.

3- What easily accessible bacteria would be able to ferment soy curd in a pleasant way?

Fermented tofu/bean curd has been made for centuries. There are number of fungii strains commonly used to ferment bean curd. There are likely many other bacteria that could be used. Lactic acid bacteria could feasibly grow in tofu/bean curd, though I'm not sure a sour tofu is something I'd personally enjoy.

Aside from blocks of tofu/bean curd, there is also a long history of fermented products with amazing flavor that use soy beans but not in a curd/block form. There is tempeh, soy sauce, koji, miso, etc.