Yeah, don't do it.
Here are the two main reasons.
1) A 5 Gallon Bucket is a little too small for a sump. See if you can at least find a cheap used 10g or even 20 for sale. Or even those sterilite storage containers (You can get a 30g one for like 15-20 bux, depending on location. Some dollars stores have them too.)
You need to make sure that your sump can handle a fair percentage of your setup's total water amount.
2) You'd have to precisely and accurately make sure that the flow of that one big pump is completely equal to the total flow of the two smaller pumps. And that's pretty much impossible. Anything even slightly different than "equal" will only cause a flood. Not to mention, you can't guarantee that the flow of the big pump will be exactly split in half if you tee it off.
Same idea with the hand vac, except now if there's a power outage, your aquarium will empty itself into your tiny bucket.
Look into overflows. Overflows make sure that your aquarium is at a constant water level, pouring out any excess water that your pump flows back into your sump. So in effect it will pour an equal amount as your pump (assuming it's large enough) and no more, no less.
And if there's a power outage, the overflow will stop, since your pump will not be putting more water into your aquarium (without power.)
You can buy Hang-on-Back overflows, make your own (out of acrylic or PVC-- check the threads) or even get the back of your tank drilled or something.