No. The Bubble-Head Charm is only intended for the caster. A similar spell is the Ebublio Jinx which is intended for targets rather than the caster themselves.
Protego Maxima is not a effective counter during a duel. Protego Maxima will conjure a magical protective barrier to cover the area, spells cast towards it will explode. As the spell takes time to conjure, the spell cast towards you would be within the barrier thus no defense is made and why it is not practical for use in a duel.
Finite won't work on transformations, conjurations, and forbidden curses. Finite will neither end confundo as that is a mental state. Furthermore here are some expanded guidelines for ending spells. Finite Incantatem will end Charms, Jinxes, and Hexes under level 35 as well as Curses under level 10. Exsolvo Incantatem will end Curses Placed by Imbue Curse (Angorem Expande). Additionally Exsolvo Incantatem will end all listed Curses, Hexes, Jinxes, and Charms and other Curses (such as those in plots or off-screen) with RPD approval. Transfigurations can be countered by Reparifarge in the case of Critical Fails/Failures. Successful Transfigurations can be countered by the spell used to perform the Transfiguration initially. Note that this does not apply to Vanishment and Conjurations.
Any spell that makes the target DO something, or have an effect on will often wear off. Spells that make alterations won't wear off, and will need a counter spell.
If a person is under multiple spells does it take one cast of Finte overall or one cast per spell the person is under.
Finite is governed by different factors as previously mentioned however to simplify if Finite is to be used, it would take individual casts.
Bombarda causes whatever it is cast at to explode. Generally, this is not useful on living things however it has the potential to be lethal if someone is close by, and should be used with caution. Bombarda Maxima, however, is more violent and is capable of demolishing walls, again it's not intended to be used on living forms, however, it can still cause injury for anyone close by.
No. A single casting will affect just one of your senses.
Does the Pepper Breath hex allow the spell's -caster- to breathe fire themselves or give their opponent uncomfortably fiery, hot breath?
The Pepper Breath Hex could indeed be cast on the spellcaster themselves. But rather than have a cool dragon breath weapon you're going to end up coughing flames with a parched throat!
They will work on any lock provided the spellcaster is advanced (or persistent) enough to overcome whatever protections might be over the lock!
Does Ferula require a broken bone to work or could you use it on any limb regardless of lack of injury? Does it work on animals as well?
The Ferula charm is very versatile! I could be used to mend a wounded animal. Additionally you could use it to splint and bind a limb with no injury, remember it also relieves pain! In this way it might be a nice way to come up with some bandages and sticks fast in a pinch.
Does Erecto only work on things like tents or can it be used to create structures out of loose materials like sticks, stones, fabrics? Could it create a bridge?
The Erecto charm needs components that are meant to work together in order to assemble them. To put it another way: If the items you had are intended to form a predetermined structure than it should work on them. Size might also be a factor. A tower, large ship, or bridge might take multiple wixen working together. An interesting use of this spell can be found among the more domestic minded wixen of the world. When ingredients are properly set up and arranged this spell can be used to combine and prepare the recipe. This use of Erecto is , of course, ineffective as a shortcut to potion brewing or artifice. Of course heating and baking would require the use of a hot air charm in a pinch, though, an oven is usually preferred.
The first thing to understand is that this spell is meant to raise and lower items vertically. Moving them side to side or otherwise would require the Locomotor Charm. While there is no definitive method to determining the weight that the charm can affect it's important to remember that there's a reason students start with feathers! Lifting a chair with a wixen standing on it would require the caster to lift their weight along with the chair. Futher unlike the Hover charm you would have to keep constant concentration to maintain the effect. Lastly, there's no real indication that the charm is capable of 'bracing' an item as such. So in the 'chair' example it's likely that it would be quite wobbly and might tip over! Make sure you keep your balance!
Does the shrinking charm, reducio work on human parts? As in, can it be cast on a persons hand and shrink it?
Yes, the shrinking charm can be used on specific body parts but it is risky to do so without a high degree of proficiency with the spell.
Can the scouring charm be used in duels to make the floor slippery for your opponent, or aimed for their mouth to wash it with soap?
Yes, but while this is a creative use of the spell, it may not be the best defence. There is no guarantee your opponent will slip on the soap if they are standing still.
The short answer is 'yes'. However this could come with dangers both to the animal itself as well as the caster. In general it's best to never 'Accio' more than you can hold. Especially if it's a living thing!
How does the Backfire Jinx work, that it has two so different spell effects, depending on whether it's used offensively or defensively?
The Backfire jinx essentially has one effect: It causes a spell to be redirected. The difference between the two is redirecting the charm 'manually' in the case of a defensive use, or, letting the jinx lay in wait for it to be triggered.
If someone casts before you and you attempt to defend with flipendo are you automatically hit with their spell? Is the only way a first year able to defend themselves by dodging?
Unfortunately, you can't defend with flipendo. If someone has already cast their spell, it's just not feasible that you can cast a spell to push them after they've already released the spell to block it. While we know that often many actions could happen at the same time in real life, for the purpose of roleplaying it's not so simple. That said, naturally first years are going to be limited. They won't be as skilled or knowledgable on dueling spells as professors focus on teaching the basics before teaching dueling spells. The primary options for them will often be sparks and that's part of being a kid. Learning as you grow and becoming more dangerous in time.