Which lens for low light event + portraits?

I would stick with a prime. You may think you'll use the zoom, but I have a feeling you'll probably end up finding a distance you are comfortable with and shooting from that, instead of adjusting your zoom with each shot.

Since you'll be shooting low light and you aren't confident in the steadiness of your hands, you'll definitely want some form of image stabilization, otherwise you will miss out on shots. Really, your best friend is going to be flash. I'd recommend getting a speed light to help with AF and lighting, so you can get those crisp low iso shots that you will want. If you aren't comfortable with an external flash, then I'd say to rent it ahead of time and do some work before hand to learn the ins and outs.

Out of the lenses you've posted, I'd recommend the 24mm 1.8 with image stabilization. Though I would aim for something closer to 35-50mm focal length. Remember, you are running a crop sensor, so all of your focal lengths will be might shorter than advertised. A 50 will act like a 35 and a 24 will be something like a 15-18. I'm a Nikon guy, so I don't know if that knowledge translates directly to Canon, but I don't think there is too much of a difference. I had the math backwards for whatever reason, but I stand by my advice.

Good luck and happy hunting.

I would be looking more at flash guns as lighting outside in a garden at night is probably not just dark, but really bad for photography.

A f/1.8/lens in the 35-50mm bracket should be fine when paired with a flash. I wouldn't rely on Tue cameras own flash if it has one though, a cheap hotshoe mounted flash will be a lot better.

One thing to keep in mind is the R10 has a APS-C size sensor. That means that the effective focal length of those lenses will be higher than on a full frame camera. I believe Canon uses a 1.6 crop factor for their APS sensors. So, while a 24mm lens is very wide-angle on a full frame camera, it will be only moderately wide on your camera.

For portraits, a moderately long lens usually produces more pleasing results than wide angle. Wide angle lets you capture more of the scene around a person, but then it's more of a landscape that includes people, not a portrait. A portrait should focus on just one (maybe two) subjects.

Of the lenses you mentioned, the 15-35 is probably your best bet. It's a "L" lens, which is Canon's top-of-the-line. It's fairly bright.
You might also look for Canon's 50mm f 1.2 or f 1.4 "L" lenses. With the crop factor, 50mm is comparable to 80mm on a full frame.
Your subjects will fill the field of view. You might have to take a step or two back, but they'll look better than when shot with a wide angle lens close up.

Longer fast primes can give you that tiny depth of field that gives that lovely separation between the subject and background. Pair with a bounced flash (assuming you're indoors)* and you can get gorgeous images.

Downside is if you're in a smaller area, a longer lens can make it tricky to frame up properly (you may find yourself trying to become one with a wall). A shorter zoom can give you a bit more flexibility (forget a big range zoom, they're too slow and you won't use half of it).

*Read it back, you're outside! A diffused flash (ideally off-camera) can give you that nice separation closer up even with a slightly slower lens. I would try the 15-35, and a flash with an extension cord (and a diffuser attachment or bounce card!). Flash can also ensure you get a nice shutter speed (dunno what the shutter sync is on a R10, but you should be fine with the hand shake), and maybe a decent pair of gloves in case it gets cold as the night goes on!

Sounds like someone needs a nifty fifty!

Camera stuff

It depends on what you're trying to achieve. Are you doing candid portraits where the people are in natural positions and not looking at the camera (that's the preferred style for things like corporate events), then you want something longer and as fast as you can get, no flash. If you're going to have people posing in front of you and look at the camera then I'll join the crowd and recommend getting a flash, with canon you might have a setting for "natural" or "ambient priority" to reduce the whole flashiness of it. In that case the 24s are probably fine, they'll be a 38mm equivalent on your body, so not actually that wide and should work for single portraits or groups (zoom with your feet).

Here are my favourite lenses or combos to cover events, but keep in mind I do this professionally and definitely cannot enjoy the event as a guest at the same time:

• 15-35f2.8 + 70-200f2.8
• 28-70f2
• 24-70f2.8

Personally I would never cover an event with primes as they just don't offer me the flexibility I need but again different situations, in your shoes I'd probably grab the 24 and enjoy the pleasure of shooting around the 35mm length, my favourite prime length.