Thank you for writing here.
If you feel numbness and tingling sensations around the legs, it may be because of issues with spinal nerves catering to the legs. So, I do not think jala neti would directly target those tissues where you feel numbness and tingling sensations. Other parts are not explained so that I can think over it. Please write back to me for this.
Yes, jala neti will definitely help you for respiratory allergies. In fact, I would suggest you to practice sutra neti also for nasal/ respiratory allergies.
If you focus on internal cleansing, kunjal is quite good as one of the shatkarma. You may also add vastra dhouti for the cleaning of the digestive tract in its full length as well as respiratory tract. No doubt, the respiratory tract will also be benefitted by kunjal.
Please check with the physician if you have any issues like spondylitis or sciatica to correlate with the tingling or numbness sensations.
If you need any further clarification, I will be happy to answer or clarify your queries.