I have done a few mixes for a friend, unpaid, and I've learned that when you aren't paid that you tend to care less, or if it's a passion work than you care more. I've gone as far as replace entire drum sounds, add parts in, delete parts, etc.  However, you don't always have that liberty, and replacing/adding sounds can take way longer than if you just mix what's pre-existing.  If I'm putting my name on it than I tend to care a lot more. If it's a work for hire, and you don't get any credits on it, I'll work with what you have.  I will say that's it's awfully annoying to have stems where every drum is in mono down the center or synths have ugly reverbs that cannot be removed without creating artifacts.  In these situations it helps to know all your tools very well, but it will only ever be but so good.

I'm not against referencing at a higher volume. But for me I do a majority of my mixing at a fixed volume. 85db isn't all that loud actually.  I keep it at -10 on my symphony.  Towards the later stages I might turn the knob a few db to hear if everything checks out, but I rarely listen at super low volumes because that can be very deceiving. I'm not sure people listen to music at low volumes very often, and if they do they aren't paying attention (background music).  If it sounds good low-medium to high it's usually a good mix.

That's just my method.  Whatever works for you to get a good mix, all power to you.

I could never fathom using a sample pack for sweeps and impacts. First of all, they sound out of place unless you build the song around them.

Impacts - it all depends on if it's a naked impact (nothing else going on), you can drop a huge bomb here, but if it's placed in the middle of a mix you aren't going to hear the full effect unless you clear out some space. And it all depends on what's going on around it or where it is in the song, because sometimes a tom drenched in reverb is more appropriate than a kick, sometimes percussion does the trick when all else fails. The standard is using a kick from your track and using a huge hall setting with 50-100% wet, adjusted to taste. The tail could be timed to the track (60,000/BPM for quarter notes), this allows you to have the tail trail off appropriately according to the number of measures or bars you need it to. Or sometimes you just go by ear and bounce down to audio and then time-stretch that shit, which can be an effect in its own right. Man I could go on for days. Don't be afraid to layers well. One thing to note is that if you stack reverbs on top of one another it can get messy, even with surgical eq'ing, so best to try and group them all to a bus and apply reverb there, but again I suggest you experiment.

Sweeps - white noise is pretty standard. You can sweep the frequencies through it using a band pass, bounce it to audio and modulate the pitch. Do a combination of both, or whatever you want to do. If you can do elementary synthesis you can program a pitch riser in damn near any synth. Once you get better at it the sky is the limit, including looping vocals and making the pitch rise, or increasing the cut off filter on a plucky synth. Could go on for days

All in all I would suggest you critically listen to tracks you love and try and figure out what the supporting rise/impact/downlifter elements are, why they are there, and what role they have. Trying to imitate is always the best way. The most organic way to make these sounds is using elements already present in the track but if you can manipulate a sample pack to work, all power to you. I just find that they are drenched in reverb that may or may not be in line with your track. That's my gripe with most sample packs actually (zenhiser, vandalism, vengance). Or maybe I'm just a control freak.