#1 see:
http://responsetype.com/shop/fairlady-z ... -adapters/
#2 more trials happening on other chassis ... going to spread the product line to toyota and mazda

#3 weld the circumference where the tube used to be to the adapter
#4 sometimes just grinding the gusset down, sometimes egging the coilover bracket, worst case a heavily modified bracket
It appears that megan racing coilovers fit without modification, but the brackets used by cx racing (same as megan's thread pitch) won't... so you could go down the path of modifying, or you could swap to megan brackets to use them directly.
The rollcenter adjuster kit is included with the adapters specifically to enhance driving dynamics when lowered, but also gives you more trackwidth; it drives up the cost, but it's so worth it...