Be advised that the following contains a lot of speculation on question number 4, as will all other posts on this topic. The short answer is we just don't know for sure.
I won't touch question number 4 with a ten foot pole in terms of decisively answering it.
Be aware that:
The "advisedness" of using handloads in a self defense shooting has never, ever been an issue in court, and any advice to use handloaded ammo or not use handloaded ammo, has never been put to the test in those courtrooms. So despite advice pro and con, it's never come to pass that it has mattered one way or another. There's no legal precedent for advising one way or another. So in giving such advice, everyone's guessing, and oftentimes that advice "feels right" to the adviser.
FWIW.
Many are absolutely adamant that handloaded ammo is not to be used. A better question to answer for oneself might be "What do my handloads offer that a factory load does not?" Is it possible, or even conceivable that I might be put into a position where I might have to use handloaded ammo? Is it possible that my handloaded ammo is not as good as factory ammo in some respects? If it is not as good, why am I using it?
An answer to that last question might be as simple as "it was available" but only you can answer why that is so. The reliability of handloaded ammo is in direct proportion to the amount of effort the handloader expended in making sure everything is correct, checking how well that handloaded ammo goes into and out of the gun, and how consistent and functional it was and whether it developed the needed velocity and accuracy.......read: experience. A new handloader doesn't have a lot of that in starting out, and it may be questionable just how reliable and accurate his ammo is given that lack of experience.
With factory ammo there's less doubt in that respect, and it's often marketed for a specific purpose, which is self defense. Can you absolutely say the same about what you're doing?
I'd doubt very much that some people would draw the distinction between handloaded and factory ammo should, say, a centerfire rifle be used instead of a handgun. Or a shotgun, for that matter. Maybe because of how damaging they are regardless of who loaded it.
Interesting that this might be the case, no? But then, I don't know that either......it hasn't been an issue in court to any documented degree.
In portraying a handloader as a "blood thirsty killer", which seems a bit lurid, one must think that such an assertion would be damaging only if the shooting was unjustified in some respects. In such cases it's hard to posit that using factory ammo would save you, as opposed to handloaded ammo damaging your case of legitimate self defense when other actions connected to the shooting were questionable. Your "bloodthirstiness" will come to the fore, if that's the case, by your actions and how others perceive you, more so than simply ammunition choice. Being dressed like a mall ninja or being festooned with "tactical" gear would no doubt be just as damaging, if not more so, if I can be excused for speculating like everyone else. Or choosing a "big" gun with factory ammo rather than a "small" one with handloaded ammo.
Remember Winchester's infamous "Black Talon" hollowpoint handgun ammo? Surely a handloader would have to work at it to come up with ammunition as sinister as this factory ammo was portrayed to be! In this case a handload could certainly generate less attention than the hysteria that surrounded the introduction of this commercially available ammo. Perhaps we should carry more tame ammo like ball so nobody gets really hurt?
I suppose the real irony is "degrees of bullet humanity." Odd concept some subscribe to.
You can download either 38 or 357 brass. In having less airspace, the 38 Special brass may give lower velocity spreads with light loads, but the bullet has more jump through the chamber before it gets to the cylinder throats. Only you can tell if there's a significant accuracy difference, and given how difficult a J frame is to shoot it may be hard to discover any differences in accuracy.
There is no "single best" powders for these two cartridges in making practice rounds. There are only powders that work, and those that do not. Magnum 357 powders (H110, W296, 4227, 2400) work less well in the 38 or not at all in low power loads. H110 and W296, especially, are not to be loaded down. Powders that are optimum in 38 don't generate full velocities in 357 loads.
Powders that work in both but not obtaining full velocities in 357, as an example of only a few:
Unique, W231, Bullseye, HS 6, Titegroup, and many others.
Cheap bullets that shoot well are cast lead bullets. Most likely you'll want some variant of a 158 SWC to standardize on a bullet weight a fixed sighted revolver is regulated for. Review the commercial lead bullet suppliers like Oregon Trail to see what they offer. Usually these are much more reasonably priced than jacketed bullets, and easier on the gun.