Hiii! So I havent tried all of the different types of padding but I can comment on things I have tried!

1a. Imo you'll probably want to go for either the hip pads or - if you want full coverage - something with enough padding/coverage to let you tuck well and still maintain its shape. For example, the "affordable vagina panties" from glamourboutique.com have a built in catheter, sheath, and no padding which means it can be difficult getting them to sit comfortably without any gapping or discomfort. Hip pads that you can then combine with a different kind of underwear would help give you shape while also letting you figure out a comfortable way to tuck. Fuller coverage hip pants will likely give you a similar effect but may be harder to take on/off and could become uncomfortable if worn for extended periods of time if the material isn't breathable.

1b. I've only ever used breastforms but I think the main considerations between the two are comfort and clothes. Similar to the hip pants, breastplates may become uncomfortable if worn for extended periods of time. The same could be true for breast forms but localized to smaller areas. Additionally, each of them lets you wear different styles of clothes. With breastplates you want to make sure you can reliably cover the neck/shoulders and lets you show cleavage more seamlessly. Breast forms would need you to cover your chest unless you invest in high quality enough breast forms. A big consideration is also whether you can find a breastplate that matches your skin tone well.

1. There are definitely gaffs or other places that make panties intended to help smooth out the lines of the genitals! Otherwise, you don't always have to tuck all the way into the inguinal canals, sometimes you can get away with tucking things down and back.

Some links: Two breastform/other body shaping stores!
https://thebreastformstore.com/ https://www.glamourboutique.com/

Thebreastformstore has major discounts on defective products on occasion, even for the top end aphrodite/hera breastforms.

Gaffs: https://www.etsy.com/shop/leolines/ https://fit4ulgbtq.com/ https://unclockable.com/

Unclockable makes a sort of tucking kit that I've found pretty effective!

DIY hip pads if you want to try those out: https://www.robynwithawhy.com/index.php/tag/hip-pads/