I have a Canon 6D (MK1), 3 YN 600RT speelights, and the YN RT Controller. I do portraits and some event photography (birthdays, baby showers, etc.). If you are asking about how the YN equipment has performed, then I can say that it has worked wonderfully for me and my needs. The cost savings are substantial. Prior to "upgrading"/purchasing the YN RT speedlights, I had a couple of non-RT Canon 430 EX speedlights, so I am somewhat familiar with the Canon speedlight family.

If you are asking about the focus assist light, you'll have to determine how important/valuable that feature is to. I have not used it because it's too distracting. The low light autofocusing on the 6D has worked fine for me thus far. FYI - the YN RT controller is compatible with Canon RT speedlights also.

But separate from the equipment questions, I do have a concern with your overall situation. Are you being paid for this "indoor thing"? With you having never worked with flash before (on-camera or off-camera), not having time to do research, not being familiar with the equipment, preparing to spend $1000+ for Canon flash equipment, etc., all seems like a disaster in the making. If you are expecting to be paid for this event, I suggest you pass this one off to someone who knows what they are doing, and you spend some time learning about how to use the flash and practicing with family and friends before charging for your photographs. Learn about bounce flash, flash power, ambient light vs. flash light, flash synch speed, flash distance, etc. I'm not suggesting that you need to have a "PhD" in flash photography - these concepts are definitely digestible and learnable with practice, and you should be able to produce pretty good pictures fairl quickly. However, jumping into this with family or clients depending on you in your very first outing with flash is very risky. At the very least, maybe you can arrange to have a primary, experienced shooter do the event, and you can be an unpaid 2d shooter getting some practice in? Good luck to you!

EDIT: Add'l answer: Whether you need two flashes (1 on camera, 1 off camera) or 1 (on camera) really depends on what type of shooting you expect to do at this event. Pro wedding photogs sometimes use both. With the OCF option, you'll have the extra burden/work of having to move the flash/stand around all night, depending on where your subject is. This might be easier if you have an asst to move the light/stand for you. I think you could work quicker and have more flexibility if you just stayed simple and went with on-camera flash (w/some type of bounce or diffusion). In any event, the YN RT controller on your camera can control the off-camera Canon flash if you wanted to go that route.