There are a couple ways to remove old sealant. You can try mineral spirits and see if it cuts into it. If not I use stripper or MEK will work.
VTS make a silicone remover that works pretty well, it's a little different, but works good on old silicone. Also 3M makes a SR RADIAL BRISTLE DISC, try 4 discs of 3" in size on a 90deg. die grinder and it will take old silicone off.
Some have even used gasoline (Although I've never tried that and must make the disclaimer that I personally wouldn't do it), but some say it works.
Personally have never used Vulkem for the install(since been discontinued)
Howerver I do have some Trempro 625, although it sounds like some are
using Trempro 626.... different cure time.
Don't use butyl for panels, use the Trempro.
If the rivet hole is 1/8" diameter, then you can drill the hole out to 5/32" by using a #21 drill bit, and then installing AN470AD rivets
SOLID ALUMINUM RIVETS - UNIVERSAL HEAD - AN470 from Aircraft Spruce
After the hole is drilled, deburr with a 6fluted 100deg. counter sink.
Typical sizes for rivets (5/32" buck) are 470AD-5-4 to 470AD5-6 for exterior, but I have size 8 just in case and cut them down. No more than 1.5 diameter sticking out the back. AD have a greater sheer strength and should be used for support. (A) Rivets are soft, so only use on something not structural.
Hope this helps,