I've just been refreshing my memory of the details of the Salisbury Convention. Although the Lords is expected to pass any bill that was included in the government's manifesto, amendments are permitted on the second reading, provided they are not wrecking amendments. Let's recall what the Tory manifesto actually stated -
"A new Scotland Bill will be in our first Queen’s Speech and will be introduced in the first session of a new Parliament. We will implement the recommendations of the Smith Commission..."
So there is no way on Earth that amendments on the second reading of the Scotland Bill that seek to ensure that the manifesto pledge is actually delivered, and that the Smith recommendations are implemented in whole rather than just in part, can possibly be considered wrecking amendments or fall foul of the Salisbury Convention. In theory, any Lords amendments could still be reversed by the Tory majority in the Commons, but the government would be faced with a very difficult presentational job if they try to overturn the will of the House of Lords, the Scottish Government, the Scottish Parliament AND almost every single Scottish MP at Westminster.
What are the chances of the Lords making those amendments? Here is the current composition of the upper house -
Liberal Democrats 100
Plaid Cymru 2
Crossbenchers & Independents 210
Church of England Bishops 26
Labour and the Liberal Democrats between them have 312 seats out of 789, which is 83 seats more than the Tories. Although it's not an overall majority, in practice it's very close to being one, because non-partisan peers only turn up when they feel like it and don't vote as a bloc anyway. Labour have indicated that they will seek to amend the Scotland Bill to bring it into line with Smith (at least in respect of welfare powers), and I can't see any particular reason why they wouldn't do that in the Lords as well as in the Commons. If the Lib Dems vote with them, those amendments will probably pass.
It should be noted at this stage that the Lib Dems said in 2005 that they no longer even felt bound by the Salisbury Convention, partly because the Labour government of the day had been elected on just 35% of the vote. The current government was elected on a near-enough identical 37%.
So we won't need to ask Tim Farron whether the Liberal Democrats' supposed "century-long commitment to Home Rule" actually means anything - Lib Dem votes in the Lords will tell us all we need to know, one way or the other. They've got no excuses.