All three main nadih intersect at every main chakra. That's why the 7 chakra (and pineal gland) are so important. The solar chakra is the most powerful because all 144 thousand nadih intersect there. These are just channels. There is no use to empower them, as far as I know. Better just keep them clean, so your energy can flow properly. The twin serpent meditation is the only thing you need for them. It cleans them and balances them (and the chakra), which is all you need to move to the next stage which is the merging of them. But merging the 3 main nadih happens automatically at a certain point. Another important thing for the balancing and purification of these meridians is the alternate nostril breathing.
I also agree with Ol Argedco Luciftias in not using that much fire. You do not need the AGNI mantra in the susumna at all anyway. That meditation has other uses than vibrating mantra in it. It is supposed to be quick way to warm you up, among things. 111/216 rep of a mantra are far from quick, even if done improperly (=at speed). You can also warm yourself up with the breath of fire, which is also useful to purify all of your nadih.
Ida and pingala do not need to be activated. They are already active, but not both at the same time in most subhumans. Generally, most will have 1 active and 1 dormant. If you you put your index finger below your nose, you can understand which one is more active by the amount of breath that comes out of each nostril. The nostril that exhale the most air is the most active.
On the other hand, the susumna needs to be activated. This only happens when the ida and pingala are fully balanced. Some text associate an allegory to this stage, saying that ida and pingala 'die'. This is obviously not a real death. What happens is that both stop working when they become completely balanced, and the susumna starts working in their stead.
It appears to me like you're trying to complicate your life needlessly.
You can find other purification methods in the money spells PDF. Scroll through it until you find exercises that deal with your nostrils.
I hope this was helpful.