Ok if you don't like the heater and if it not adjustable i would return it and order the one that you like before in that link. It up to 5 gall meaning you can really use it in your 2.5 gall and it adjustable. It hot now and you really don't need the heater now. Unless you want to check it fires and it will work great. What kind of the heater. I thought all heaters which is for tanks bigger than 1 gall were adjustable. You really can keep the temp in your house about 75-76* and you don't need the heater until you order a good one. It sucks though that the store don't have it.
What is pH reading? Bettas are very adjustable to the pH. And what is pH in the water that he is now? If it the same then he already get used to it.
Not sure why he is spitting out the pellets , he might be just adjusting to the new food. Do not give him blood worms yet. He might like tham and will not eat pellets. So wait until he will get used to the pellets first.
And do even smaller water changes then i told you. Do like about 6 small water changes about 25%. Just let him get used to the new water.
Now i am not sure about pH drops. Did Sakura told you to use it? I don't know if it good to use the drops to lower pH. And again like i said check the ph the water that he was in all the time and new water see what is the difference.
Sorry i think we posted at the same time i didn't see your post