Yes, and ?
Pretty much, except that because you can't be certain of your perceptions or experiences, you can't be certain that it's true. That is the problem with synthetic propositions.
You may think you have experienced rain falling, but you can't be certain that is the case - you could be dreaming, hallucinating, misperceiving some other phenomenon, etc. Having an independent opinion may increase your level of certainty, but given our tendency to perceive what we expect to perceive and corroborate reports (e.g. rain falling), doesn't necessarily increase the likelihood that you are correct.
In practice, you generally assume your experiences reflect reality unless shown otherwise.
Yes. Perceptions and experiences are mental events that are not infallibly reliable, and also the results of sensory input, observations, and measurements that can be flawed, and may not necessarily reflect reality. It's not my argument in particular, but an argument that goes back to the ancient Greeks (restated by Descartes, Locke, Hume, Berkeley, etc.).
You can have degrees of certainty about synthetic propositions, but not absolute certainty.
[any experts in this field are welcome to correct me if I've gone astray here]