A few inaccuracies here.
Skill ups when desynthing an item are possible . . . it is also much rarer to see skillups from a desynth
Agreed. I can't remember the last time I actually saw one, but I don't really desynth very often anymore.
Desynth recipes cap at the level of the recipe for the same item
This is generally assumed to be the case, but I don't know that it has actually be proven to always be true. I know I've seen a number of recipes on wikis over the years that claim a different level for synth/desynth, but I have never attempted to verify or otherwise resolve these discrepancies. It's a good rule of thumb, however.
and the results are only based on what goes into making the item.
A good rule of thumb, but not always the case. The only specific counter-example I can think of right now is Angel's Flute, which is made from rosewood and parchment. Although it has since been changed, the desynth result used to include the possibility of an angelstone. This may have been an unintentional exception, but it is (was) an exception none the less. There are also a number of items which can be desynthed, but for which there is no (or no known) synthesis recipe (sky gear, for example).
If you pile a bunch of Moghancement furnishings together though, you will receive Mega Moglification, which gives +5 skill. At minimum, the 150k furnishing, signboard, and guild stall are required for this to work.
I think the guild stall alone gives Mega Moglification, but it'll still only be +1 skill without other furnishings.
This was ninja-changed such that the stall alone gives +5 skill. I have personally tested this and verified it to be true (at least +5, I did not attempt to rule out higher values), and others have claimed such over at BG.