Questions concerning Binance margin trading - Shorting / Borrow / Repay

Hi, I have a few questions concerning Binance margin trading (for shorting). I have no problem with spot trading and get good results on long positions so I wanted to try shorting. Remember this is just a small test before getting real on short positions.

So I tried to short RLC/USDT using Binance margin trading. I transferred 15$ from my spot account to isolated (x5) RLC margin account. I then borrowed 1.5 RLC for roughly 15 USDT using (auto) "borrow" with SELL button to short it. I adjusted my TP and SL using an OCO order and aimed my TP get me around 15.5$ from the borrowed RLC.

The TP was hit, so I got my 15.5$ but then needed to repay the borrowed RLC. So what I did was put a long position on 1.5 RLC using (auto) "repay" with BUY button. I adjusted TP and SL accordingly and my TP was hit, so all good I presumed. But what happened is that I still had a little RLC to repay (I guess I didn't take into account fees or something like that ?). Note that I didn't "maxed out" the RLC amount when buying to repay, because it was a x5 isolated and didn't want to buy more then needed.

You can see on the attached image that I still owed around -0.0021 RLC.

I then saw the manual "repay" button, so clicked it and tried to repay, but had no more RLC to do so.

Because I didn't want to be "in debt" (even for such a low amount), what I did was buying more RLC and then used that manual repay button.

Questions are :

1. Why by using the (auto) "repay" function with BUY button, setting RLC to 1.5 (borrowed amount) didn't repay all the borrowed amount, even tho I made profit on the whole operation ? If it is about fees or something like that, how can I set the RLC amount to make sure it will repay the whole borrowed amount ?
2. Would I have been better off using the manual "repay" button to repay the borrowed RLC, instead of putting a Long position with (auto) "repay" ?
3. Same as above but for borrow, would it be better to use the manual "borrow" button to borrow the desired amount of RLC ?
4. What would have happened if I didn't bought more RLC to repay manually this minimal debt (-0.0021 RLC) ? By the way, note that all the interest fees where paid, the remaining debt was from the borrowed amount minus the fees.

Thank you very much for your help

https://preview.redd.it/ighgucs937z61.png?526&format=png&auto=webp&s=d291a86164c224d0f4537c16413deb8fb87ce600