1)Possible, but not in the method you want. It is fairly easy to make things transform themselves (i.e. have a bronze colossus that transforms into a steel colossus in battle) but very difficult to have them be transformed by other things (such as your dragonfire example). You will want to take a look at the
interaction token page on the wiki, as well as the werebeast example interaction in the raws.
2)Possible, but it's hoops as fuck and can only take items (i.e. the metal bars) if you as a adventurer/fortress do it. Wild creatures or enemies could potentially summon something, but can't have any requirements for it (so it could summon colossuses but wouldn't need bars) other then corpses, which are a special case.
0)At this point if you wanted the reaction to require something you would cause your reaction to take the required items. If this is a temporary thing then the reaction generates a boiling stone that enables the interaction in step #1. Natural abilities can't require items and go straight to step #1. For a summoning that takes corpses you can move straight to step #2.
1)Creature transforms itself into a 3-part creature, a main body, a grasper, and a connection part. The connection part should have a boiling temperature below room temperature. This transformation only needs to last a couple of frames. The end result should be your creature as before (this will fully heal them!) and a grasping body part lying on the ground.
2)Creature uses a raise dead interaction with a very short range that is only allowed to target their specific type of 3-part creature. This interaction raises the grasping body part, and then transforms it immediately into the desired creature. They then lose the ability to do this interaction.
3)The end result should be the starting creature who will be fully healed due to transforming, and a "clone" creature that will be immune to all future transformations (due to being already transformed).