adding key sig and time sig to start of each line.

Style->Edit General Style->Page.
Check "Create Clef for all systems" and "Create key sig for all systems".
That sorts out the key sig and clefs on each line.

As for the time time sig - not usual to have it at the beginning of each line but you can manually drag a time sig over to the first measure in a line (or any other measure, for that matter) if you wish.

This is not exactly the same question, but I'll shoehorn it in here. Is there a way to create one key signature and one clef at a regular interval (other than by hand?). For example, I would like to have the clef and the key signature either at the head of every page, or every odd or even page (for two-sided printing).

Context:
While unusual in general, this is common enough in rehearsal scores. I'm editing a violin répétiteur from a 19th century ballet and I'd like for the MuseScore file to look as much as possible like the original, without laying traps for the next user. It has the clef and key signature at the top of each page.

Similarly, I've seen this crop up in contemporary jazz ensemble and Broadway conductor's scores. The scores are dense, and the space saved by omitting key signatures and clefs every line is valuable. However, the only option I see in MuseScore "once each time it occurs"..... but I don't think that's what's actually meant when we see a lead sheet. When there is a more hefty score, the clef and key signature appear at least once in the visible space. There's no point being in the middle of a score and not being able to double check, the only question is how often does it need to be printed. Top left, either of each page, or each page turn.

Sorry if I missed it in plain sight. I don't see how to do it without either hiding a million clefs and key signatures, or reasserting each manually, at the top of each page..... which will be messsssssssssssssy once I've finalized the reference score, and begin changing the layout (it's 300 pages).