(33% more Defense and 50% less Speed? - Similar to the revamped Weak Armor)
50% more Speed to 33% less Defense is because those are the inverse of each other -two thirds vs three halves. It's exactly identical to what happens when Weak Armor activates in normal play: one stage of Defense down is 33% less Defense while one stage of Speed up is 50% more Speed, which is in turn because that's how you maintain the proportions when flipping around. 50% of a stat is the reverse of double the stat -not 50% more of the stat.
So if Battle Armor is to be made the reverse of Weak Armor, it would be 50% more Defense and 33% less Speed.
Should we give all 4 of the armor abilities a critical hit immunity?
Battle Armor at least, should.
Are any Pokemon more viable now thanks to the revamped version of these abilities?
Battle Armor: Drapion might actually bother with it now, since it has trouble outspeeding things anyway, and its Ground weakness is primarily a physical vulnerability anyway. Armaldo might also bother with it outside of rain teams, and same for Kabutops. Marowak technically appreciates it, but it's not going to launch it to OU viability.
Magma Armor: Doesn't save Magcargo, but Camerupt will finally be able to survive
one Water hit, at least in Sun conditions. Doesn't address its real problems of being slow and neither particularly tough nor particularly hard hitting.
Shell Armor: For all the Shell Smashers, can be a bit of a nasty trick. Who needs a White Herb? Protect being able to effectively double as a Cosmic Power is nifty. On the other hand that's half your moveset. Shell Armor would certainly help some of these Pokemon in Doubles play, where Protect shows up on almost every Pokemon regularly. Not sure it would really bring anybody up in viability per se though, being a bit of a narrow gimmick, particularly from a Singles perspective.
Weak Armor: Choice Scarf, the Ability will definitely help a lot of the Pokemon that get it -Omastar is actually a good sweeper, normally held back by the need to set up with Shell Smash or have Rain support, and indeed most of these Pokemon are fast enough that, at least with Speed investment, they could outrun 100+ Speed Pokemon reliably, with just a sacrifice of taking 50% more damage from Physical attacks. Mandibuzz probably won't care for it and Magcargo is so slow that it would basically have to Shell Smash
and run Weak Armor to have any chance of outrunning anything at all fast, but most of the other Pokemon are just slow enough they can't pull a straight fast sweeper but fast enough they don't need much assistance to pull off the task. Variant Skarmory running as straight sweepers might actually happen, as opposed to always running Sturdy and defaulting to being all about hazards. Even Garbodor might see some use, since it would be a lightning-fast Poison type -Crobat being the main Poison type that isn't on the slow side, and it has its own issues, such as vulnerability to Stealth Rock.