1. My understanding is that you can have 2 bitcoin wallets within the same ledger. correct?
Yes, if the wallet using HD wallets
[1]. And if the wallet software has multi account feature.
2. if so, assume that bitcoin is $100. You transfer 1 BTC from wallet A to wallet B within the same ledger. is there a transfer fee? if so, roughly how much?
Yes, bitcoin transaction require a fee, unless it's an exchange and it is called internal transfer.
Regarding the fee, it depends on the network
(considering you just bought the $100 from an exchange with a single input) higher fee if congested network, smaller fee if the opposite.
But first, transfering coin from the same wallet is not reasonable it is a waste of fee, you will also see a warning doing that.
3. still assume bitcoin is $100. You convert this to tether. a month later, you can convert this back to BTC. assume BTC stayed at $100 the whole time. Roughly, how much fee will I be hit with?
Base on the number 2 answer, it will not be $100 anymore. Regarding the conversion, it still a fee so depends on the conversion fee. Now regarding the fees, it still depends on what platform you use to convert. Fees are dynamic, so no one can tell if how much is it on the time of transaction and conversion.
[1]
https://learnmeabitcoin.com/technical/hd-wallets