MGB= Mass Group Buff
Gained by assigning AA points (Alternative Advancement experience points earned after level 51). It means that you can cast a beneficial group buff and instead of targetting just the group it will affect everyone in the surrounding area. It costs twice the normal mana (or all your mana if you don't have twice).
LoY introduced quested spells for all casting classes at level 44 and level 52. Some are better than others. You would have to use the spells listing on this site to see if yours were worth getting.
LDoN introduced quite a lot of spells. The Reebo ones you mention are for inspecting, disarming, and opening chests in LDoN adventures. There are similar ones for other casting classes. Opinions vary widely on whether these are worth the cost, especially at lower levels.
There are lots of other LDoN spells such as Spirit of Shrew - an indoor SoW, Blessing of Temperance - a group version of Temperance and so on. They are all bought using adventure points so you need to do quite a lot of LDoN before you can get them (depending on your level - a 200point cost is 4 adventures at 65 but likely 20+ at 30)
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