Yes to every question.
If you’re going to put the rear wheels on algae or ice, lock it into 4×4.
Putting it into park with 4×4 engaged holds the front tires too. Setting the parking brake is a good practice to protect your parking pawl.
If you watch videos of vehicles being towed out of the water, 95% of the time it’s on an algae ramp in 2wd and IN PARK (possibly even the parking brake set too). If the rear wheels lose traction without a foot on the (front) brake, she gone….lol
If your vehicle isn’t 4×4, it’s best to launch with two people on slick ramps so someone can always have a foot on the brake (front brakes engaged). A front wheel chock wouldn’t be a bad idea either.
Ice isn’t nearly as bad as algae (when it comes to losing a vehicle). The ice forms out of the water on the ramp. You can have a scary slide down the ramp, but when the back tires hit the water they grab and stop you since there isn’t ice under the water.
Algae is the opposite. When your back tires hit the water they can keep sliding if your front wheels are free wheeling.
Thankfully the two don’t usually go hand in hand. An ice covered ramp out of the water with algae under the water wouldn’t be a good combo lol