Hi babuyogi,
My mom arrived last month with wheelchair assistance even though she can walk.. She does not understand english and but she knows to write. What i had done is i printed out an example copy of immigration form at port of entry. You can just google and print that out. I wrote it out for her with all the informations and sent her a scanned copy, so that if there is no one to fill up the form for her, she can copy out of that. And thats what she did.
Even though you have singed up for wheelchair assistance, if your mom stands and walks away from her seat, there is no way that they will know and chase for your mom. So just ask your mom, even though other people walk out, don't panic and just stay on her seat. At the end, they will come with wheelchair to get your mom. And they take directly to the port of entry counter, i think they have a separate counter for people with wheelchair assistance. The same person will take her to baggage claim and your mom will have to show them, her luggage. They wait for you till you meet your mom, they wont leave her alone so don't be worried. It took my mom around 20-30 minutes to come out after plane landed Just tip him $10-$15 . P.S. my mom did not bring any food, i dint want her to get in trouble.. just in case...I hope it helps...