One answer to the question above is the infamous West Lothian Question. The Westminster Parliament is three separate institutions messily rolled into one. (For the sake of simplicity, I am going to ignore the existence of Northern Ireland.) For matters which are not devolved to the Scottish Parliament, it acts as the UK Parliament. For matters devolved to the Scottish Parliament, but not to the Welsh Assembly, it acts as a Parliament for England and Wales, while for matters devolved to the Welsh Assembly, it acts as an English Assembly. Yet all MPs can take part in any debate and vote on any issue, and it can be possible for Scottish MPs to tip the balance on a vote on legislation which will not apply to Scotland. Some people see even the possibility of this happening as unacceptable, and there have been mutterings about constitutional changes to resolve this issue, should there be a No vote in the referendum. (I find it ironic that the opposite situation, whereby English MPs could, by virtue of their greater number, out-vote Scottish MPs on matters relating solely to Scotland, caused so little concern that it was allowed to persist for nearly three hundred years.)
The danger with any approach which removes the right of Scottish MPs to participate in certain debates or votes in the Commons is that it could reduce Scottish MPs to a lower status, and make it more difficult for any MP for a Scottish constituency to be appointed to certain ministerial positions, including that of Prime Minister. The result could be that Scotland would be under-represented in the Cabinet, which is arguably the real UK Government. There is also an argument that some things which do not directly affect Scotland can still have indirect consequences; for example, cuts to the NHS budget in England will reduce the Scottish block grant through the Barnett Formula, perhaps forcing the Scottish Government to make equivalent cuts.
This constitutional messiness could, in theory, have been avoided if devolution had been implemented more thoroughly. One solution would have involved creating an English Assembly, with exactly equivalent powers to the Welsh Assembly, and a Parliament for England and Wales (PEW), to deal with all matters which are devolved for Scotland but not Wales. The PEW could consist of the two Assemblies in joint session, meaning that AMs would handle exactly the same range of business as MSPs. Video conferencing technology could mean that Welsh AMs would not have to travel to London. With the resulting large drop in the workload of the Westminster Parliament, the number of MPs could have been cut and the House of Lords abolished, freeing accommodation for the English Assembly in the Houses of Parliament.
There is one obvious reason why this was not done, and probably could not have been done. The creation of an English Assembly would have required (not constitutionally, but in practice) a referendum in England to approve it, and it seems very unlikely that approval for such changes would have been given. I can just imagine the howls of protest against the abolition of the Lords, and the ending of hundreds of years of English tradition to suit the Scots and the Welsh. Let the Lords remain, and the protests would be about the increase in the number of politicians.
Thus the current devolution settlement stayed clear of any changes which would have provoked a demand for an English referendum, and the West Lothian Question is the result. This has implications for the possibility of any significant increase in the powers of the Scottish Government following a No vote. Radical constitutional changes which require to be validated by an English or UK-wide referendum are probably even less likely to happen in the foreseeable future than they were in 1999, given that surveys have shown increasing opposition to devolution in England. Without such changes, additional powers would exacerbate the underlying problem and increase the likelihood of Scottish MPs having their status downgraded. Arguably, this is rather academic, as there is very little chance of any significant devolution taking place, if the threat (to the British Establishment) of Scottish independence is removed.
Opinion polls have consistently shown that devo max, whereby the Scottish Parliament would have full authority over everything except defence and foreign affairs, is a popular option; superficially, it appears to be a sensible compromise proposal. If there was going to be a second question in the referendum on devo max, it is almost certain that the result would be No to independence and Yes to devo max.
It has been suggested that the Scottish Government never wanted a second referendum question, but did not want to be seen to rule out such a popular option. So they said that they would consider having one, knowing that the unionists’ knee-jerk reaction would be to oppose anything which they thought the SNP wanted; David Cameron duly delivered the desired result as part of the Edinburgh Agreement. I find this view plausible, and I believe that the Scottish Government showed good judgement.
Devo max is fatally flawed; it cannot be implemented in any sensible way without radical constitutional changes which are very unlikely to be approved by the English electorate. A vote against independence and for devo max could have been disastrous, as Westminster would have been unwilling, or even unable, to deliver the necessary constitutional changes to make devo max a viable solution. My guess is that the UK Government would have put a bill before Parliament and then let it fail, while insisting that the No vote on independence must stand.
This links to a second answer to the question I posed in the title. As long as Scotland is part of the UK, Westminster trumps Holyrood. The powers of the Scottish Parliament are limited to those allowed to it by Westminster, and Westminster will never forget that there are about ten English voters for each Scottish voter. UKIP wants to see the Scottish Parliament turned into a committee of the Commons, and it is possible that they could be future coalition partners with the Tories, many of whom probably see the Scottish Parliament as a nuisance; neither party has much to lose by offending the Scottish electorate. Only independence can ensure that the Scottish Parliament is not stripped of some of its existing powers or even abolished.
The present devolution settlement is much better than what went before it, but it is not an adequate substitute for independence. I hope that in the future the verdict of history on Scottish devolution will be that its main benefit was that it enabled the people of Scotland to take control of their own nation.