You cannot rationalize with him, so save your breath and your energy. There are some things you can discuss or explain to a child, and there are some things that you can't. Sometimes the kid needs to learn that YOU are the boss and he's outta luck.
If he doesn't obey the rules, then he doesn't get to go with you is a good place to start.
If you carry a purse, switch to a backpack so you have both hands free. Grab on to him tightly. If he writhes or bites you, restrain him---sit down if you have to and hold on so he can't bite you.
If he is biting then he loses all priviledges for a bit---no tv, etc. Pick something---what would you do if he bit under other circumstances?
IMO Running away in a parking lot is far too serious an issue for a typical "time out".
Whatever you do MUST be so unpleasant that he will never ever want to run off again---whether it is no dessert or losing a favorite toy, or a smack on the butt, the consequence for taking off MUST make a huge impression.
Ditto for the car seat---do you still have his baby car seat? threaten to put him back in the "baby" seat if he doesn't stay buckled.
Look for a car seat that he can't unbuckle by himself. Contact your seat's manufacturer and see if there is a buckle that is harder to undo.
Also, keep the straps snug and the center part up by his armpits.
**Put him in charge of making sure everyone else is buckled...maybe he'll like being the boss!
If he unbuckles while you are driving, pull over as soon as it is safe (be dramatic about it if you can) and start giving him holy hell. If he has a toy, take it away from him. Scare him with the yelling about how unsafe it is. Strong emotions anchor memories, so make him feel very sad and he's less likely to forget the lesson.
Can't the 10 year old help? Have him buckle when he gets in and make a big deal out of it---have him tell the little one that big boys always buckle, etc.
With my kids we had rules and consequences and yet the safety rules were always non-negotiable, heavy punishment things. Running away in a parking lot, unbuckling, things like that called for the "nuclear weapon" level of punishment. There was no room for excuses, explanations or pleading about "safety rules." Even today now that my kids are a bit older they know that if I say it is a "safety rule" they had better obey it.
****But if you threaten a punishment you MUST follow through.******
One of my kids had to skip the last five houses of trick or treating at Halloween one year (his brothers got to go on) because he broke a safety rule. To this day my kids remember that and they know I will follow through with what I say.(Even if it makes ME sad.)
Good luck, I know how terrifying and exhausting this can be.