Not entirely sure I understood your first question, it sounds like two questions, so I'll try to answer "all three" if I'm understanding you right. Please note, there is still much that's unclear so I'll try to cover all the possibilities.
1)I'll use some basic algebra for this one. Each Toa has 100% Toa Energy normally. Having this infused in your body is what makes you a Toa. Yet, each single Toa can make six new Toa. Thus, that energy has to be somehow multiplied at some point during the process. So the maximum potential Toa Energy from each Toa is 600% at minimum.
And it's vital to note that making six Toa Stones doesn't immediately turn you into a Turaga. That only happens if you use the rest of your Toa Energy for something else, or if six Matoran use those Stones. In the latter case, the rest of your energy seems to "teleport" to them. However, the one time we saw this done, a Suva (a teleportation shrine) was clearly involved. It's possible there are multiple ways to do it -- maybe you CAN put the full amount in the Stones and become a Turaga instantly, or choose not to, in which case a Suva might be necessary.
Either way, the total amount of Toa Energy multiplies by at least six (plus apparently a tiny remainder in the Turaga).
Now, a Toa Stone is an object infused with Toa Energy. So to do this, they must take out some unknown percentage -- X% -- for each Stone. They can do this six times, so they drain 6X%. At that moment, they may be left with "100% - 6X% = R".
What happens next is unclear -- there's all kinds of possibilities. X (the amount put in a Toa Stone) might be almost one-sixth of 100%, or far less (or both may be options). The Toa might be stuck with whatever that momentary remainder is, or the remainder may multiply to bring the Toa back up to full until the Stones are used, etc.
I think it's most likely that at least a few minutes later, the Toa will have 100% Toa Energy. Lhikan remained a Toa and had full control. I don't think there's such a thing as "half a Toa". The key here is that even though they must subtract some energy, this energy also absolutely must multiply anyways, so it's quite plausible that it begins multiplying as soon as the process starts rather than at the end, so the original Toa will remain at 100% until the moment they become a Turaga.
So I'd say no, it doesn't weaken them, but that's just my theory.
2) (Re: "must they become Turaga") No. If nobody ever uses the Toa Stones, and they never use the energy for something else (like the Toa Metru did), they'll remain Toa forever.
3) Let's keep in mind that Toa would be immediately needed throughout the Matoran Universe (since the Matoran population was rapidly brought up to full, to help construct the interior of the robot), and it would be inefficient to rapidly spread a whole population by Toa-izing someone only to have them turn to six of their neighbors and immediately Toa-ize them. That just wouldn't make sense. The exact math of this is hard to pin down, but there are two key theories that seem most reasonable to me:
First, that there is probably a Spawner machine (as I call them) that makes Toa, the way there are multiple ones that make Matoran. This is probably located on the island of Artakha, made either by the Great Beings or him, right away. So this could be used however often is necessary immediately to spread Toa around.
Second, that the GBs and/or Artakha probably made devices like the Toa-izer beam on the Red Star down in the MU also, so it's quite possible that some Matoran could have been immediately Toa-ized elsewhere, without even needing to make a Toa in Artakha and travel all that distance to where they were needed. However, then we must wonder what happened to these devices if this is the case and why we've never heard of them. I suspect that if they made any, they probably made only one or two and put them at the Southern Continent and/or the other arm that doesn't have Artakha in it. Then later the Order or someone else could have hidden them.
One or both of these could be the case.
Also note, it's highly unlikely that anybody is descended from any of the eight Toa who are confirmed to have been made as Toa, since all eight are still Toa. It's possible if less than six were made from them, but that would seem unwise IMO.
I like this theory, but didn't GregF once say that Toa Energy and Elemental Energy are different? Meaning, over time, a Toa can improve their control over Elemental Energy allowing them to use less and less for the big, speeding up the recharge time, but Toa Energy doesn't have a recharge. Once it's used, it gone and won't come back. Toa Energy makes the Toa, but Elemental Energy is the thing that gives them power.
Anyways, we only have the story of Lhikan change 6 Matoran into Toa and the Toa Metru creating six stones as a back up to protect the Matoran on Mata Nui. So who really knows how much Toa Energy a Toa has until it is used up.