If you do try to open the switch to see if it can be cleaned/repaired, put the thing in a sizeable plastic bag along with yoiur hands. This will save you grovelling about in all the detritus on the floor looking for the small ball bearing and spring(s).
I once did just that for a switch on a circular saw that lived upside down in a table. Lots of very fine sawdust had clogged it up. I doubt this switch is going to be good enough for more than a short life since I think it has probably worn out due to repeated arcing. Weed trimmer switches get a hard life as people are stopping and starting them constantly.
If you are going to spend time on this ( I get it) I'd at least start with a new switch to help make the effort worthwhile.
In Australia it is also peak season for people throwing stuff out after buying lots of new stuff for christmas. Keep an eye out for power tools and particularly leaf blowers. I found a corded one recently that had a torn collection bag. I really just wanted the heavy duty 30m extension cord. I found at least a dozen over the years just on my short drive to the supermarket and they ALL worked. I think people just "upgraded" to a cordless one. Same goes for vacuum cleaners. I found a top of line Hoover that was clogged up. I think they wanted a cordless and just weren't going to empty it one last time. That was 5 years ago.
It's a trade off between needing it fixed now and storing extra stuff just in case it comes in handy later. But even if I do eventually return it whence it came (roadside) I keep the brushes, switches and anything else that takes my fancy as potentially useful but not too big.