Have the Kingdom do slightly better in its wars against both Scotland and England in the 900s so that native rulers are more frequent and allowable as vassals.
Have Duncan I of Scotland die young before he's crowned this will remove Northumbrian/English support of Malcom and Donald in the civil war against MacBeth. MacBeth may still have a Northumbrian wife like Duncan but there will unlikely be a fullscale attack against "Scottish" Strathclyde.
Strathclyde could then act a bit like Wales in occasionally having to acknowledge England or Scotland as overlords but with the bonus of having a willing ally against the other in maintaining its separation. Though a 3rd possibility is it being split into 2 - A Scottish Clyde and an English (greater) Cumbria.
I think the easiest way for this to happen is to break the string of Alban successes in the 10th and 11th century that were neccessary for it it to evolve into Scotland.
If the Albans never take over Lothian or Moray, then it's quite possible the balance of power can be maintained, and Scotland remain broadly disunited.
Then the challenge would be keeping it disunited with England, though.