Streetwind

If you want to use different mods (or no mods at all) in different worlds, then you need to toggle them on/off in the mod manager in the main menu before joining those worlds.

You can potentially buy them from other traders and sell them for profit? To be fair, I've not checked if that works with cracked vessels, but there are several other trade goods with which that works.
Additionally, the Malefactor has a chance for a cracked vessel to drop itself instead of its contents. That way you can make a few extra gears on vessels which no longer contain anything interesting to you.

Inside your VS install directory, go to ...\assets\survival\entities Open item.json in a text editor, and chenge the despawn time to your liking. It must be given in seconds. Save and close. Run modmaker.exe inside the VS install directory. Wait and follow the instructions until complete. Open item.json again, and change the value back to the default 600. Save and close. You now have a mod that changes your item despawn time to the value you want. It is likely already where it needs to be, and you'll be able to activate it from the mod manager in the game's main menu.
If you suffer from performance issues when running that mod - well, redram already explained why But in singleplayer it should be okay, so long as you don't choose an utterly ridiculous value.
If you just want to keep your stuff from despawning when you die, by the way, it would be way easier and less problem-prone to just set /worldconfig deathPunishment keep instead of fiddling with despawn times.

Sounds like the chunk reset. That should definitely not happen.

Not really a bug, no.
I can't really tell you how precisely cellars are coded, as I cannot read the code. But I can tell you how it works in practice in the game. Cellars are not truly 'rooms' - you don't need to build a completely enclosed space of certain specifications. No, they work through proximity of stone-type or soil-type blocks. Each storage vessel, chest, shelf, and so on will check around them in a radius of as much as five blocks away, to see if it can find valid cellar blocks. The greater the ratio of valid blocks to invalid blocks, the slower food spoils. The slowest possible food spoilage rate is achieved if the storage vessel detects only valid blocks - that is, there is a wall, floor, and ceiling made up of valid blocks in any and all directions, and none of them is more than five blocks away. It can be much closer - for example, a vessel buried underground by itself essentially occupies a perfect cellar of size 1x1x1. But if it's further away than five blocks in one direction, then it'll detect only air that way and it will no longer be perfect. This is why people tell you that cellars should be 5x5x5 rooms. Not because there's actually a room setup requirement, but because that ensures that there's valid blocks nearby in all directions.
By removing the valid blocks in the floor and replacing them with invalid blocks (wooden slabs), you reduced the effectiveness of the cellar significantly.
If you feel like you storage vessels aren't updating to the correct value, try picking them up and putting them back down.
 

Pretty sure there is no random spawning mechanic either. I've played for multiple ingame years in worlds with zero wild crop respawns.
You just sometimes come across a plant you missed the first five times going through that area. It happens. Just like it does with surface copper deposits - where you return from your three thousand block copper scouting mission only to discover the deposit right next to your base that you completely missed all this time.

Not a bug, working as intended. Every item maintains its heat for roughly one minute after the last time its temperature increased, provided that nothing else updates the item's temperature in the meantime. Not just in the firepit, but with all heat sources. This is supposedly so that you can do something with the item you just heated before it cools down again, for example smithing a tool on the anvil.
Updating the item's temperature through any means waives this grace period immediately. For example, you can heat a stack of 30 unfired bricks to 600°C, and then pull it out of the fire. It will maintain those 600°C for about one minute. But as soon as you merge one other brick into that stack, the stack's temperature will be recalculated to be the average temperatue of all 31 bricks in the stack. This modifies the temperature, and so the grace period is waved, and the whole stack starts cooling down immediately.
You can actually use this behavior to exploit the heck out of the firepit. For this reason, the roadmap already has a rework of the firepit earmarked. And who knows, maybe the temperature system as a whole will get overhauled at the same time? Or not. We don't know. Nor do we know when it will happen.
Until then, however, this is not a bug.

Welcome to the forums
You can save the ruined bloom until you have a windmill set up that drives a helvehammer. The hammer can work blooms for you, so you don't have to do it all yourself. And it will produce a perfect ingot every time, even when the original bloom would not have enough voxels for it. So it will actually repair a bloom that you accidentally ruined.
It will also produces plates from ingots, and plates can be used to start chainmail recipes, so the helvehammer is a great help when producing heavy armor. Every workshop should aspire to have one!

You're supposed to be able to turn the ingot into chunks with a chisel... I believe by crafting them together.
 

When you take the crucible out of the firepit before the contents melt, the contents will spill out onto the floor. Depending on how your firepit is built/situated, you may not see this.
Just like all other items, if these bits are not picked up within 10 minutes, they will despawn.