1. Don't need to make a starter with dry yeast, but it works if you do so no worries.
2. Re-using your yeast cake would be fine as long as your not using it for a lighter coloured beer. Even though you didn't make a starter as long as you had a nice healthy fermentation that yeast should be in perfectly good health if not amazing health after multiplying and eating all that good sugar. Imagine it like the beer you just made was a 5 gal starter for that liquid yeast.
3. Use your starter and Harvest your yeast, or save your starter and use your yeast cake.
Hope I understood what you were asking and answered your question.