Aw, my dd was also born 2/09. But, I still call her "the baby." She's well into toddlerhood I guess; she's been walking since just before Thanksgiving, and is a climber and runner now. But, she's still so young and little...she's a baby. It may help you to remember that your little guy is still really, really little, and there's so much about his world he just doesn't (and can't) understand yet.
1. How long do I feed her? Well, I work really hard to avoid two things at the table. The first is a battle, the second is a bad habit. So, she sits at the table until she starts to stand up, or goosh food, or whatever else. Then, with very little comment, maybe "are you all done?" with the sign, I get her down. Usually, she cries because she isn't really done, she just can't sit still anymore. So, we hold her on our laps and feed her more. When she is reasonably full, she is happy to be put down, so she wanders and plays, coming back for a bite now and then. Eventually, she's not too interested in coming back (we don't push it, btw. Just ask every now and then if she wants a bite), and we are done eating, so we put stuff away and we're done. At this age, it's still whatever works. You have lots and lots and lots of times for manners and habits and such.
2.) How do I get the groceries in? Lots of ways. Dd1 would not have tolerated being in her carseat or the house alone for any length of time at all. Sometimes I would wear her, or carry her while I just did one bag at a time. Sometimes I would put all the groceries in our wagon, while holding the baby or letting her play on the ground near me, and then pull it to the door. Then she could be right with me just inside the door while I unloaded into the house. Then I would shut the door so she couldn't get out, and carry everything into the kitchen. Or, I'd just wait until she was asleep and do it then. Now that I have 3, and a more laid back baby, I can just leave them in the car while I carry stuff in. Sometimes I still do the wagon thing, and sometimes I still just wait until later. When I sense I'm going to need some time to get to it, I put all my cold stuff together and ask the bagger to put it in plastic. That way I can easily spot which bags are the cold stuff and be reasonably sure nothing will go bad in the car. On mild days, I just don't worry about it much.

. You could try giving him a snack in his carseat. A tiny box of raisins keeps my dd2 busy for a long time. She also loves those little snack cups that don't spill very easily. Maybe he'd be distracted long enough for you to carry things in. I always leave the doors open so that they can see me. Oh, and sometimes I just pull up to the door and leave the kids in the car while I unload and then go park in the driveway. It's much faster. I don't usually carry things all the way into the kitchen or whatever, though. I just put all the bags on the porch. I've been known to leave them there all afternoon, too!
3.) Yeah, a lot of them are scared of the potty. They often think they are going to fall. I'd just shelve the idea for a while. But, if you really want to do it, try putting him on a potty seat AND turning him to face the lid. He'll feel much more secure that way. Also, you (without a potty seat, and facing forward) can sit behind him with your arms around him.
4.) Between 2 and 3, my kids have started noticing and preventing messes when I point out what happens when they x. Like, oh, look, when you sit like that, the milk can fall off your spoon and gets all over your chair and your shirt. Lean over your bowl like this and that won't happen. See, the milk falls back in your bowl now! They don't like getting messy (usually...); it just happens. Dd was always a very neat eater. Ds has never been. He had tomato sauce all over the place today after lunch...but, not on his clothes. Just all over his face, hands, and arms. It's an improvement. Until they have a little control and restraint not to fling it all over the place, I only let them self feed foods that aren't awful to clean up. Today, dd fed herself rice cakes, while I slipped in bites of mashed potatoes. She got to feed herself, and I got to know she got some real substance and didn't smear it all over the table. And, since I've perfected the art of feeding the baby with one hand and eating with the other, I got to eat lunch at the same time as everyone else. I always give her something to eat by herself, even if I plan to feed her the messy stuff in between bites. Oh, and my kids always eat messy stuff (no matter who feeds it to them) wearing only a diaper.
The more children I have, the younger they seem. They really are still so little, and are learning SO much. Just brace yourself for the journey and enjoy ever second. It passes really, really fast.