Working with the carburetor is not too difficult. Re-sealing the gasket at the cylinder head is easy with the correct sealant. I recommend Hylomar, available at most any auto parts store. A little goes a loooong way. You only need a thin coating on both sides of a clean gasket. It is easiest to apply with a q-tip dipped in acetone used to spread it around.
When it comes time to disassemble the assembly in the future, it will just come apart really easy, usually with the gasket in perfect condition to re-use. Nothing much to scrape clean and no burnt gasket stuck on to scrape and scrape and pry off.
Silicone sealant on the other hand is a pain to work with comparatively speaking.
The fuel will not damage the case seam gasket in any way. It might make the very edge swell up a bit but that will not harm anything.
Have I ever adjusted the float lever?
I have been rebuilding small engines ranging from weed whackers to Harley Davidsons for 27 years, so yeah, I have adjusted a float lever or 100 thereabouts.
16:1 is waaay too much oil to break these engines in with.
The Chinese print the instructions as though you will be using straight 30wt automotive oil, which is common in China for these engines, not proper 2-stroke pre-mix like we have. 24:1 is very good for break in, 32:1 thereafter, speaking for common good quality pre-mix oil. Switch to 32:1 after the second gallon or 400 miles.
To convert 16:1 into 24:1 add one half gallon of gas to the fuel mix you all ready have.
I recommend getting rid of the cheap pre-mix fuel you have though.
Add it to a full tank of gas in a car or truck, it will not hurt anything.
If you choose to use a high quality synthetic such as Opti2, follow the instructions carefully for that type of oil. It mixes at 100:1 and that must be followed exactly.
Cheap pre-mix oil like what is found in Target, K-Mart, Walmart, ect is not horrible for these engines but is really is not good either. It certainly will reduce the overall lifespan of the engine, but it is better than automotive oil.
Your spark plug color does not sound bad, considering you are burning 16:1 fuel.
You will have to re-adjust the needle clip position after switching to the higher pre-mix: fuel ratio to keep the carburetor dialed into the desired air/ fuel ratio.
Move the clip up one notch at a time untill you get a nice chocolate brown color on the spark plug. Test often to insure that the engine is not too lean or damage can occur.