to your first question. no, we all use the same nodes. but it is possible to download an incorrect block chain from them as there is a fork after the last checkpoint in the wallet.

second question, you cant do that. the wallet source doesnt know to read checkpoints from anywhere besides the hardcoded ones. this means that in order to make an external text file like you said i need to add that into the source anyway and just adding checkpoints is less code than what you are suggesting. so it is not possible without source modification anyway. also any addition of checkpoints means you would have to redownload the block chain reguardless of if it is in source or a txt so it doesnt matter.