Okay, i felt like i really have to answer this question being that i'm also Vietnamese.
I have two kids (a 4 and almost 2 year old). Also, I was raised here so I consider myself pretty Americanized. However, I would feed my children Vietnamese foods FIRST if I had to choose between that and American baby foods in a jar (lots of preservatives, unless it's organic..and even then..who knows). Porridge is a very common food that not only Vietnamese, but Asian cultures give their babies when their starting to learn how to eat solids. Porridge, although is made of rice, does not consist of rice ONLY. We Asians eat rice with everything (but not rice ALONE as some here may think Asian porridge is). In fact, if he was eating only rice, he wouldn't be pooping that much. for sure!
For babies just learning to eat, a common meal would be porridge, cooked with eggs (steamed) and tender, ground pork. For vegetable, you mix in pureed carrots or squash, something soft enough to go with porridge. This is easy for the baby to swallow and digest. The idea is for baby to get enough iron to grow big and strong. That's really how most Asian grandmothers think in terms of health. And honestly, it's really healthy. They get the protein, carbs, vegetables in every meal. No preservatives, no additives.
But don't see it as porridge everyday. It's really rice with different meats and vegetables. Asians eat rice almost EVERYDAY. We don't see it as eating rice. We're just eating different dishes and rice is like tortilla to a hispanic dish/meal. So your baby is really just being fed the Asian way, except that your baby is eating "soft" rice with the "sides" of meats and vegetable.
I mean, I pretty much eat rice 4-5 days a week. I al ways make sure my kids have meats, vegetable and carbs for lunch and dinner. I rarely ever give them sandwiches or PBJ for lunch. In fact, my kids have had PBJ once in their whole lifetime. It's not that we don't like it, it doesn't occur to me to make a sandwich for lunch. I always feel like I need to cook a full meal. Not mac 'n cheese, or sandwich with chips. Those seem like fun/snack foods to me, to be quite frank. But again, a cultural difference. Now would you rather mac 'n cheese or a full meal with meat, rice and vegetable?
And in regards to the grass jelly. I haven't looked at the ingredients, but I grew up eating it and all i can say is, it's not sweet, nothing like those "high fructose corn syrup" jello that we find in the local supermarket.
Now in regards to MIL not respecting your wishes, I do agree that that is wrong. But please know that MOST LIKELY it's not a personal thing to you at all.. It's a cultural thing. She really thinks you worry too much. I promise you that. And she's really not trying to disrespect you. I really believe that majority of the time, that's the case.
But I do understand. You should still try to have your husband talk to her. But limit it to something very specific like..when the baby's older, ask her to not give baby any candies or cookies without you guys' consent. I think that would be something she can do.
But in the meantime, I think your MIL does care about his health, that's why she gives him these foods. AND cook for him everyday! And I mean cook, not open up a box and mix things. Real, from scratch cooking. I don't get that kind of support with raising my kids because my father passed and my mom didn't raise me. So all we have is my MIL..and she is great, but she's still working full-time, so I really had to learn how to cook everything like the way I was raised. I have to make sure my kids eat healthy with the little talent that I have in Asian cooking. I used to feel so jealous of my friend who's mom would prepare all the meals for her and her sister's baby. And i used to think, man i wish my kids could always eat all that natural foods. But I had to rely on baby foods in a jar when they were babies because i was still learning..figuring things out. Not that it's completely unhealthy or anything, but not exactly fresh and who knows about the preservatives. But I quickly learned to cook better. And i try to cook Asian foods and non-Asian foods too (from Chinese to Moroccan), but they have to be from scratch, not out of a box with powdery mixture. Yet even then, there are occasional times when I still feed them Mac 'n Cheese when i'm crunch for time. i have them sometimes as emergency (but never felt great about mixing that powder stuff in with the macaroni - nothing about it look natural). I wasn't raised eating it, so always felt anything like that..Hamburger Helper...isn't meant to be real food. Sorry, just don't mean to offend anyone, but that's my perspective. Although i've lived here ever since i was 3, I guess my thinking is still very Asian when it comes to foods.
Also, you won't know what it feels like to have a 8 month old go 5x a day (as long as it's not diarrhea) until you meet a baby who doesn't go at all. Know that a good bowel movement means a very healthy child.
In regards to the juice. One thing i know about Asian moms or grandmothers..for the majority that I've grown up with. They are very diligent of always making sure that the baby is properly hydrated. That's why they always give lots of fruits and juice. My MIL always squeeze fresh oranges for our kids. And peel apples and pears for them when they come over. But I would definitely ask your MIL to dilute the juice with water if it's not fresh squeezed. And believe me, a good bowel movement is a big deal to us. If baby is pooping regularly, we believe it's a sign of good healthy. Just make sure it's not diarrhea.
As far, as nap and a schedule. I agree with you. As I am the same way about that stuff. But I did read the others' perspective and i think it depends on the mom's philosophy on this. So I do encourage you to put your son on a schedule and tell your mom that this is the time you want him to nap. At his age, I say he can nap around 9:30/10:00 and again around 2:30/3:30
good luck.
btw, i seriously laugh when I read a comment about how it's not a cultural thing but an uneducated thing. And how you need to get books on nutrition..(perhaps to educate your Asian MIL on proper nutrition(?)) Tell me what other cultures eats healthier than Asians? Not meaning to open up a whole can of worms. I'm sure there are. But Asian foods top the list. The funny thing is, I really do believe that no one realizes whether they're eating healthy foods or not (meaning no one is counting calories, transfat etc). Asian recipe just so happen to be what it is (goes with our palate) and fortunately, it happens to be healthy too. Vietnamese is one of the healthiest foods with lots of vegetables and, especially lots of raw vegetables.