I will answer the above questions:
I was adding power because the mixture stopped boiling in the boiling flask, sometimes it did not boil, so I had to add power. But the problem is that as I increased the heating power to get the mixture to boil, so the ester poured from the soxhlet into the boiling flask, it boiled violently...
I use a molecular sieve 3A with soxhlet to drying ester because the ester is wet, and the balance is by drying shifted to the right side. I found this method on the Internet, said to be very good for high yield of ester.
I do not use sulfuric acid because it reacts with formic acid and it decomposes to carbon monoxide. Formic acid is strong, one of the strongest carboxylic acids, so it catalyzes itself.
What to try to pour formic acid with a little methanol into the boiling flask, heat it up and slowly add the drip of the methanol according to the rate of ester dropping?