1. You really only need to change the water when your nitrates get too high. In order to tell whether you "need" to do water changes as often as you've been doing them, you need to see where your nitrates are.
2. You should just follow the direction on the bottle. However, I do think it's better to dose using shorter period (daily, every other day) rather than once a week because you aren't putting too much nutrients in the water, which may allow algae to gain a foothold.
3. If you have enough of the bacopa and java fern, it may still be wise to dose the water column. If anything, you can possibly lower the dosage.
4. I don't think it's necessary to get your bubbles to spread all over the tank because that really is just the undissolved CO2. As long as the diffuser is near your source of water filtration (assuming you have a good flow), the dissolved CO2 should get distributed. If your water flow isn't that great, then you may need to get a powerhead to add more flow to your tank.
5. Ultimately, the best bet is to dose dry ferts because you can alter your dosage as necessary. I don't know that there's necessarily a best fert brand out there. The truth is that no one bottle is going to have everything you need. You need at least 2 bottles of ferts - one for macros (NPK) and one for micros (trace elements). I believe the ferts you are using include only micro nutrients. You can continue to dose them, but you need to buy a bottle of macro nutrients.
Also, I think your photo period is too long. Most aquarists use a photo period of no more than 8 hours.