Yes, it is legal to own a Minecraft client, although using one could be prohibited by servers you visit and you could have consequences from the staff. Making a client is not different from making a mod, which is totally ok to do, and it is actually supported by Mojang.
All of those clients are mods and are therefore legal to use and own. A lot of clients have features that are very useful for building, such as flight and freecam. Most servers will have rules prohibiting the use of cheats such as xray or pvp cheats, so if you want to use a client avoid those servers and stick to servers that allow them. Beware that a LOT of clients have spyware in them and you DO NOT want to trust some of these client makers. Read up thoroughly on a client you want before you actually download/buy it.
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Go read JJBA and Fairy Tail! Oh, add me on Steam as well!
Well, hacked clients are not real life "illegal" (they won't get you arrested). HOWEVER, in my experiences as being staff on a server they make my job suck. Hackers take the fun OUT OF EVERYTHING and I hate having to deal with them. I wont tell you WHERE to get hacks (although I know where) and I wont tell you how to install them. But I will say PLEASE, for the love of clam don't use them in multiplayer. Just have fun in singleplayer.