Any suggestions on how to prevent Asta and Sond being killed by spriggans if you take the sap from the Eldergleam? I can never reach them in time, as the spriggans on the way back down take too long.

There may be an optimum level where their weapons level enough to put up a fight, and the higher-level spriggans haven't started to spawn, but I can't work it out. So I'm thinking there may be a way to get them to move out of the danger zone, or something similar.

The best way I found to keep them alive is take the person from Whiterun and do the peaceful option.

I do not think I have managed to save any of them if I just take the sap.

Their death isn't scripted, so I'm treating this as a challenge. I'll probably make a save outside, so I can go back at various levels to see how it goes.

At too low a level, the spriggans will kill the player. At higher levels, the spriggans level up, too (matrons or worse). There should be a "best" level, and then it's a tactical thing. I'm assuming the spriggan spawn points are fixed, and they all spawn at the same time, but I should check in advance in the CK that two of them don't get placed next to the "victims".

One thing I may try is reverse-pickpocketing decent equipment onto Asta and Sond before going near the tree.

Maybe being able to have a companion wait near Asta Sond. Maybe also having a pet or one of the automatrons from Dragonborn DLC. Maybe they can be enough of a distraction.

I'll have check the follower idea, too. I don't remember if telling them to "wait here" stops them fighting. If they don't get involved, it won't help.

I brought that guy with me to the Eldergleam but he only said a few comments. I couldn't talk to him, nor did I get any kind of reward. it was something I actually never experienced in all my past playthroughs (that man wanting to accompany you). Was he supposed to do something?

He is supposed to get mad at what you are doing and then suggests that there is another option and then communicates with the tree and then you will get a sapling

http://elderscrolls.wikia.com/wiki/The_Blessings_of_Nature