It is ॐ नमो नारायणाय॥ om namo nārāyaṇāya || This is the most important  mūla mantra of Lord  Viṣṇu

Please share the mool mantra of Lord Venkateshware (Tirupati Balaji)

Yes, this is a mistake and thank you for pointing out the same. It is difficult to correct the video now. Let me explore the options. I regret for the mistake. 

It is a suggested practice that I have followed. I have not seen scriptural edicts for or against the practice. I used to wear a separate mala few years ago and don't do so now. I always keep my japa mala in a separate bag and use it only during my sadhana. In some of the mantra articles, we have mentioned the energizing mantra for the japa mala also, before the actual mantra recitation. In your case, you can continue to wear the mala that you are wearing and use a new one for japa, if you wish to follow the general ssuggestion. One can easily argue that someone may have created this procedure to sell more malas etc. It is best to follow your guru's advice when in doubt.

The reversal procedure is mentioned in Mantra Mahodadhi and suggested for Ari or enemic mantras. Even enemic mantras can give favorable results if the proper procedures described in the fructification of mantras article, are followed.

Raviji - Dhyana Shloka in the Youtube video link you posted above is misread. Kārmukarujā is mispronounced as Kāmrukarujā. FYI

Also on wearing Japa Mala, I ve heard suggestions both ways. Though most people suggest NOT to wear the mala used in japa anushtanam but i also heard some gurus who suggest wearing the japa mala which gets energized during japa anushtan. Based on such suggestions, i have been wearing my rudraksha japa mala for 16 years now and I have been using & wearing the same japa mala in various anushtans of different deities(male and female). Do you suggest to stop wearing it and buy two fresh malas one for wearing & one for japa? Are there any scriptural references for such injunction or is it a suggested practice? thanks.

I have heard many say that there are ways to let go of a mantra 1) uttering it in the ears of Go Mata 2)writing it on bhojpatra and offering it to a "live river". I heard these two procedures from many and most recently from courtallam peethadhipati Sri Siddheswaarananda Bharati garu. Abandoning a mantra without the above two invites misfortune (according to Sridhara Swami of Mukkamala,"mantra tyagi daridrataha")

Also, the mantra reversal technique really does not apply to a mantra that is composed of seed (bīja) syllables only. In this case, if the mantra you followed for Bālā devi is "aim̐ klīm̐ sauḥ", then there is no need to reverse it, since the mantra is only three lettered syllable mantra in Sanskrit. Only mantras that contain words, may be reversed. For example, if the Gaṇeśa mantra (om̐ vakratuṇḍāya hum̐) does not work you, then you may abandon this mantra by reciting the reverse mantra 108 times as (hum̐ vakratuṇḍāya om̐). Note that this mantra also has two seed syllables (om̐ and hum̐), but because of the word vakratuṇḍāya, the reversal can be attempted. A 108 recitation removal is okay, but no point in reciting any further, as it will become another mantra japa by its own! In your case, if you decide to stop reciting śrī bālā mantra, you may do so without doing any more recitations of any kind. Simply pray to Her and let Her know that you will not recite anymore due to the bad effects. To the best of my knowledge, if the mantra is initiated properly, its meaning well understood and assimilated and also recited in the prescribed tantric manner, it will not affect anyone. May the Divine Mother help us all in obtaining our desires and help fulfill our wishes.

I'm very sad to hear your experience. Sometimes the karmic barriers are too strong to break and by the time the Mantras get to it, we may lose our patience. Regarding your question on reversing the mantra chant for abandoning a mantra, the process described is correct. Technically, it may also apply to the kavaca and sahasranāma as well. In the normal practice, it is applied only to the mantra. Reversing here applied only to the words of the mantra and not to the letters. For example, the six lettered Bālā mantra, reverses the first three letters of the three lettered mantra. Unless you have followed the three lettered mantra, the reversing is not required in your case. It is important to realize that Bālā devi represents our ājñā, or the third-eye cakra. In other words, She is opening up our consciousness and making us realize the true nature of our existence. It is important that we understand the three letters of Her mantra and their significance. Equally important is the curse removal mantra, which is the Tripura Bhairavi mantra and that should be chanted until some of these bad effects are removed. All aspects of the mantra japa, such as the nyāsa-s, viniyoga, dhyāna etc. to state the very purpose of the mantra japa. All mantras are neutral. We state the purpose of the mantra recitation through the viniyoga, protect ourselves through the nyāsa, pray and fixate our mind through the dhyāna and align our breath to the mantra and recite. As we continue to do this, it is also very important for us to elevate our mind and state to ourselves, that we are freeing ourselves from the bad karma and we must invite the good karma and all that we desire. Doubting the effectiveness of our mantra japa, or the deity and the mantra, would invite failure. In certain cases, you should also consider the samskāras of the mantra and go through the cleansing process, described in the fructification of mantras article. Please do not burn your degree certificate or any other qualifications that you have received. It will not solve any problem. When you pratice Prāṇāyama, it is important to do this with abdominal breathing and also combine with the alternate nostril breathing. The breathing must slow down and one should try and calm the mind as much as possible, for a higher quality japa. It is difficult to do this when burdened by a lot of difficulties, but eventually, the problems will go away. Remember the adage - "When the going gets tough, the tough get going!". One most important thing about mantra japa, is NEVER WEAR the japa māla. It brings bad luck. You can wear another māla, but the māla used for japa, must remain in a bag and used only during japa. If the Bāla mantra is active in you, then you will be protected against all black magic. Sometimes, we may need to invoke Pratyaṅgira devi to counter the black magic, in extreme cases. You can recite any of Her kavaca-s for protection, especially the sarvārtha kavaca, which by Itself, can give you more than what you can imagine or ask for. My advice, be positive, think that the mantra is working and try to remove the doubts in your mind and also expel the negativity, to allow the mantra to manifest its power. Please follow the entire mantra procedure and recite the kavacas also.