Inactive Alliance Leader

If an alliance leader goes inactive is there any way to replace him/her without having to create a new alliance with a new leader?

No.
There's suggestions you can support however to implement this feature.
(Let me look really quick)
They're deleted in merges if they fulfill the requirements to be deleted, but the whole alliance disbands if that happens :/.

Here's a similar suggestion:
viewtopic.php?f=4&t=11350

Yes and no.

it happened 2 times with the same alliance in my server. the alliance is still the same, but leader did change.

i heared they asked to support to change the name of the leader. now it's one of my ex member. lol

but i don't know if the leader had to ask by himself first.

That reminds me that that's happened in my servers as well, but what happened was either the alliance leader got banned (somehow they got a new leader, probably a vice) or the leader's name was changed.
If you asked support personally, I think they'd help you with this.

Ask support nicely and they will change your leader. Has happened many times. Make sure to include who you would like to change it to and explain your leader is inactive. If someone has logged on to your leaders acc recently they will give the leader 24 hours to reply before changing the leader