Actually I've tried to figure this out for myself some years ago ... and ended up with the same problem.
A line of ten chiefs + Timmain and a total span of 10000 years does not fit together. At least not when Bearclaw is considered to be the "oldest elf ever" with 1000 to 1200 years of lifetime. And LIFETIME does not mean CHIEFTIME!
I finally decided that there must be some shaded memories ... and to make up my own "timeline" - loosely connected to some hints I gathered together.
Years of chieftime:
500 Timmain (leader)
That's my thoughts:
The High Ones story always seemed to be told in "fast motion" for me. It would hardly have happened within a few years or decades. On the other side Timmain had not too many time to bring "strong blood" to them. Depending on how many Firstcomers really were there in the beginning I figured a time of a few hundred to 1000 years.
Timmorn was a full hybrid and torn between Wolfsong and Starsong. Can't imagine that he would live rather long - though he started to do the "job" at very young age for sure.
Rahnee lived on with other elves ... not too much chief time for her.
Wild guess for Preypacer.
Two-Spear's reign was ended by a challenge - and therefor shortened.
Skyfire is shown to be almost the same age as her brother - no good start to reign especially long.
Freefoot's chieftime was remembered as a longer, peaceful period.
Tanner's chieftime was described as long and peaceful for sure - and he was described as very High One like. He was the only one who visibly aged - hair turning grey included. That's why I decided to make him very much older and grant him a overlong chieftime. (Yup, I know the Wolfrider's Guide says something about 800 years - this time I do not care about it! Had to fix this d*** chiefline somehow )
Wild guess for Goodtree.
Mantricker seemed to be an older elf when he was killed ... just my impression.
Bearclaw - I made his about "1000 years lifetime" the chieftime, supposing the Wolfriders would at least remember halfways right in his case. And to avoid messing up his legend at all.
Of course all this is open for discussion - or just an alterate suggestion. As long as we don't get a statement from Wendy or Richard on this there will be no "right" one.
on a sidenote:
There's one thing I've always wondered about ...
... wouldn't Icetooth have to be the oldest Wolfrider ever? Icetooth is from Two-Spear's time - depending on how long Kahvi was "sleeping" in the pond he must have lived 5000 - 7000 thousands years!!! Depending on her "resurrection" being some centuries or millennia before EQ1 Kahvi would join the club. (I don't count her time in hibernation.)