For a long time i just took one sim travelling - my Lara Croft sim as i love, absolutely love, the tomb aspect of it.
Lol! I thought I was the only one who did that. Only my sim was named Laurel Craft.
Since everyone else has answered your 4 questions, I'll just offer my personal opinion on traveling with/without a group: It depends on what I intend on doing there.
A lot of the time, I just hop over to the other countries to grab the skill goodies and harvestables. In this case, I travel with one sim, buy/gather everything I went there for and leave as soon as I can. I start in China first so I can train Martial Arts while I'm at home, and save France for last so I can make sure that my sim can meditate and zeneport.
If I'm going with the
intent of tomb raiding, I only travel with the sims that will be doing the tomb raiding. Typically, I use a single sim whose entire life's purpose is to open every tomb and grab every artifact they can get their hands on. But there are times where it's advantageous to do tomb raiding with a pair of sims. It can take less time when they're able to tag-team traps, and it's very helpful for getting a photo of a sim who is under the influence of a mummy's curse. Doing this is part of an adventure opportunity, and taking a photo of them getting cured of it (at least) is part of a photo collection.
But other than that, I don't take extra sims when I travel. Tomb raiding requires a vast amount of your attention, and it's way too much trouble to keep switching back and forth between the two groups of sims in order to keep both groups busy. The non-plundering group really just wastes simoleans in travel expenses.
Here's a few more general tips in regards to bugs that are related to travel:
1) Excavation: (You won't understand what I'm talking about just yet, but you'll quickly figure it out) One of the most long standing travel bugs is the inability to excavate on uneven ground. Around the world, there are numerous small sites that you can (or could if it wasn't bugged) root through in order to get many different kinds of artifacts. However, this isn't really a big deal, since most of the things you find in them can be found randomly in the tombs themselves.
2) Family Tree Issues: It's not a game breaker, but it's annoying none the less. Your family tree can get "broken" sometimes when you travel. You can also end up married to a sim you've never met. It's even quite possible for you to end up with children living with this mystery partner...even if your sim is female. As if such a thing wouldn't be entirely shocking to a male sim.
It's unknown exactly what causes it, but the easiest ways to reduce your chances of getting this bug are to not travel with unmarried sims and to minimize contact with the locals as much as possible. But even then, it's always wise to save just before you travel.
3) Adventure Boards: Sometimes the notice boards that you get adventures from don't work. This is one that affects everyone's game differently. Some players will have trouble in different countries, and it doesn't happen all the time, but your likely to see it at some point. Again, not a game breaker. The trick is to just keep clicking "Check for Adventure" and you'll eventually get one. Sometimes traveling home will fix this, however sometimes it won't.
4) Vanishing Sims: This one
is a game breaker. On
very rare occasions, sims will vanish the moment they arrive at the vacation destination. They won't be selectable, and their portraits won't show up on the left side of the screen. If this happens, it is
extremely important that you
do not save the game. Just exit and try again. This is another reason why it's always wise to save the game before you travel.
And since you're a relatively new simmer, I'm going to offer you a bit of advice that I wish someone had given me when I first started playing. If you're not already doing so, get in the habit of keeping 2-3 copies of any save that is important to you, made at different points in time, and keep back ups of them on a flash drive. Save files get easily corrupted and sometimes bugs start before they manifest. Back tracking a few sim days is much more desirable than starting over from the beginning. I have a flash drive that I use exclusively for the sims, and I back up all of my save files before I start playing.
Sorry for the wall of text lol.