Ah, I get it now...

Abra claims to be a non-custodial wallet (which is unverified), but only for BTC, LTC, BCH and ETH. For other coins they probably use bittrex's api, so for all other altcoins (except BTC, LTC, BCH and ETH), abra is a CUSTODIAL wallet.

IF what they're telling you is the truth, they are right and they cannot help you. However, there are 2 more things you can do:

• Search Abra's TOS/FAQ/... take screenshots if you have to. Try to find out if they were clear upfront that they are only non-custodial for 4 coins, and they keep all other coins on an exchange. If they did not tell you in their FAQ, their TOS, on their website or in their app, you might be able to ask them to repay you, since you were using their product under the false assumption they were non-custodial for all altcoins, so by not telling you that they kept some of your funds on an exchange, it's a little bit their fault that you cannot recover your funds
• Try contacting bittrex TOGETHER with abra's support, explain the situation (you sent BTC to a BSV address, not knowing your BSV private keys were not derived from the xprv generated by the recovery seed from abra, but instead they were generated trough an api with bittrex). Try to convince bittrex support to send you your BTC back. Do realise you'll need abra's support, since the deposit address was most probably generated using abra's api