Yes, you can reload your catches while diving in the river to try to get extra bottles or bricks or whatever you need. Some areas don't have bricks in, etc...each
diving region has different possible catches. Cherubae's guide on SoS is quite thorough so you can find all the specifics there.
Plazas areas? Do you mean in vendor area, or do you mean the extra farming areas? Or do you mean extra little sections in town where there are those signs? (Those are only for putting decorations in, though there are decor combos that can give you very small bonuses to things like FP gain with villagers, discounts from specific vendors, etc. I never found them worth bothering with. I only ever bothered with placement combos on my farm for things like protecting from storm damage, speeding up maker times and quality, etc.)
The first new vendor will not unlock before Summer 1 no matter how much you ship, but how fast you ship 100,000 is up to how much work you put into it, there's really no way to estimate a date when you'll do that...it's entirely up to how you play the game. If you're shipping everything you harvest or cook or catch/etc, you'll probably hit 100,000 well before Summer 1, but if you're hoarding everything and not making much use of cooking and so on, then it'll take you longer.
Personally, I always get the vendors unlocked ASAP. I save only what I most have to and sell everything else. I even tend to do things like buy all of Otmar's carrots and cook them and sell the dishes or sell the carrots directly. I sell all the bugs and fish and etc, and the vast majority of my crops, which I cook first if I have a recipe for them that is worth more than the crop itself would be. And I go diving every day and sell all the good stuff (like topaz) unless I'm saving them for a use I know I need them for, such as one of the makers. I find it no problem to get the first vendor on Summer 1.