>> (1) What is the consequence of accidentally leaving a small air gap?
I don't know...but, I have heard it said that an Air Gap with BP can occasion an over-pressure scenario...but, regardless, one was always advised throughout History, to compress the Charge well...so, in my opinion, one should just see to it that their Black Powder Charge is compressed well...so as not to invite troubles.
>> (2) What's the purpose of the wad?
Wads are for use in Shot Guns, or in Shot Shells, as a means of permiting loose Shot to accelerate without some of it passing other-of-it by in the Barrel, or, being left behind in the Barrel for having got settled in with the Powder...and, when used on top of the loose Shot, to keep the loose Shot from rolling 'out'.
In Handgun Cartridges using Black Powder, a thin Lubrication Wafer, or other similar thing, is simply a way of Lubricating the Bullet, reducing fouling, preventing Leading if all else is well, and, allowing an easier clean up later.
>> (3) Do you worry that some BP will fall into the primer pocket? And creating a air gap.
No...one does not worry about that.
One compresses the Charge anyway...in seating the Bullet, so, there is no looseness to the Powder, and, no where for any of it to go to create any gap.
>> (4) How do I do reduced load? Use multiple wads?
Within reason, one could seat the Bullet deeper.
A Lube-Wafer is not a "Wad" of course, in function nor in appearance...it is a means of suppling Lubricant for the Bullet, for the Bore.
Wads for reducing or eliminating an Air space...I do not know...I have not heard of it being done.
If you do elect to use 'Wads' for anything, remember please, they are to be counted as being part of the projectile weight.
Some people use 'Cream of Wheat', or other similar materials for occupying the room between a reduced Charge of Black Powder, and, the Bullet base...using good compression of course, for the charge-filler combo.
I myself have not tried this, since I have only been interested in full Charge BP Loadings.
If you want reduced Charges, possibly, a Smokeless Propellent, like 'Unique' may be an easier recourse. For which, one would best appeal to Loading Manuals, or, the favored recipes of those familiar here on THR.
>> (5) I use these "smokeless bullets" from bulletworks.com; it's has "smokeless lube". Should I remove this lube and replace it with what y'all recommend?
I do not know what the composition of that Lubricant is.
If it is a Petroleum base, I would, remove the 'smokeless lube' if shooting them with Black Powder.
Solvents, or heat, either one, ought to do it...the Bullets then may be re-lubed in their Grooves, with a right type Lubricant...or, be used with a Lube Wafer...or even both.
> The lube channel doesn't seem that deep or wide. Should I order some new BP bullets? If the latter, who sells BP bullet for target shooting and plinking purposes?
Dunno...
Casting one's own would alleviate those issues.
Or, just try them and see, but, petroleum based Lubricants may make for weird fouling and tenacious goo streaks in the Barrel, where, Natural Plant-based and Insect based or Animal based Lubricants, will vaporize and disperse and be compatable with the BP with no weird chemical reactions.
>> (6) What about the danger of working with an "explosive" as opposed to a "propellant"? Are there any special precautions?
Avoid conditions which promote Static Electricity in association with one's Loading Bench and or Powder dispenser...is about all I can think of.