There is no reason to expect a sudden reversal in the parties fortune- their problems long predate Leonard and are more fundamental than any leader, especially one who gets the press coverage of a third part, can change by themselves- but it is probably a necessary move, if you can't make people recognise you after three years you're probably not ever going to do so.
Agree. He never struck me as a bad guy at all, but fundamentally he was not cut out for any sort of political leadership - as his failure to become a household name outside his own house demonstrates pretty clearly.* Unfair to blame him for the wider problem(s), but that shouldn't be used as a shield to ward off the obvious: that he was just a bit rubbish, really.
Anyway, back in 2017 he defeated Sarwar 56.7 to 43.3, with the bulk of that margin coming from affiliates - amongst the membership it was 51.8 to 48.2. This reflected the fact that though that contest was branded a Left/Right faction-fight there were other things going on: all of the unions lined up behind Leonard, except for Community (who do not have many members in Scotland). Yes, even USDAW.
As for the successor, we shall see, I guess. Not an easy job and therefore not an attractive one.
*Disagree with the argument that 'well that's because SLAB are the third party at Holyrood now'. Nonsense. Means there's less automatic profile, but does not take away significantly from the space to create a profile, as, well, the now former leader of the Scottish Conservatives did prior to the 2016 poll.