Rainbow Fresh

According to the wiki, they do indeed not spawn in too cold climates. Which makes sense as they also do not work with too low temperatures, so in those climates they'd never produce honey.

If you haven't even tried to update yet, what makes you think you would need to fix anything, manually, trying to mess with the database of the save?
Just make a backup, update the game, update all your mods. Make sure none of the mods you use have any specific instructions/incompatibility notes for updating. Then just play. The game is very robust when it comes to updating a world.

From other posts I have seen, the do have (or can have) a surface entrance - so you will likely know when you come across one. That said, it's not necessarily the most obvious landmark so depending on the environment you might still need to look closely to find one - or find 5 right next to eachtother, like others did. Keep in mind, while the underlying feature allows for procedurally generated dungeons, right now there is only one variant in the game.

See, that's some informative context someone outside of your friend group couldn't have possibly known. It's why you yourself are best suited to make such decisions. This also seems like it's gonna potentially devolve into psychological guessing games, so not wanting to be eligle for blame I'm just gonna play it safe - the decision is simple:
If you gift the game and the person just straight up hates it, never even touches it and the money gone into the gift is going to waste. Would that be a big enough problem to outweigh the chance of the person joining you and your other friend in Vintage Story instead so you don't have to play baby's first block game anymore? If so - don't risk it. If money isn't the problem, just do it. Worst case scenario you'll just find out your interest in block games is not compatible.

All these answers are based on what I know from other discussions or wiki information, not because I changed the month length myself:
1. Yes
2. Yes - Just look at the currently set value (running the /worldconfig xyz command with out a value after tells you the current value iirc) and do some percentage calculation to even it out. E.g., if you spoil rate is 1 on 9 days, you'd want about 0.67 for 12 days.
3. Don't think so? As for something needing to be tweaked alongside, see 4
4. No. Hunger drain remains the same iirc. Therefor you might also want to adjust the hunger rate multiplier in accordance, much like the spoil rate, to account for slower crop growth.

It seems there is no option specifically for crops, or atleast I don't know of one, so your only bet is using mods.
https://mods.vintagestory.at/show/mod/44983

The mod side does not define the "supported" side, but the "required" one. A mod with mod side "client" work only on the client, and can therefor be installed on a client even if the server does not have it. A mod with side "both" is required on both server AND client to work. Meanwhile, a mod with side "server" only needs to be installed on the server and can hence change the game even if players do not have any mods.