It is unlikely that you have a negligence claim against your neighbor since you consented to ride the bike. The only other cause of action may be against the manufacturer for a possible defective throttle although you would need to have it inspected shortly afte the accident. However, your med-pay coverage may statisy the medical bills.
Unfortunately, cases take a long time to navigate through various discovery deadlines. However, based on your description it appears that your lawyer should be consulting with experts to address the loss income and future medical care your husband needs as part of the overall damage calculations.
Presumably, you filled out an incident report at Walmar. In addition, you need to send a certifed letter to Walmart to preserve any video evidence of the coverage area. Also, it is unclear why your mother fell and whether other customers avoided the same area that caused her to fall. Obviously, if it was caused by an open and obvious condition then liaility will be difficult to overcome.