Funding Interactive Brokers account- transferring USD

Hi ya'll,

I have a UK bank account + Revolut + Monzo and am looking for the best way to fund my Interactive Brokers account - open to other platforms if they're cheaper - my q's are:
a) Does it make more economic sense to just transfer my GBP to IB and then use their platform to convert to USD? I know they charge .20 basis pts for conversion + any other fees, as opposed to transferring directly in USD?
b) does IB charge for withdrawals? does my cash not invested in my account earn any interest?
c) Are there any alternative platforms better than Monzo/Revolut/a UK bank account to transfer USD to IB?

Other q's on anyone familiar with Monzo and Revolut:
Which of Monzo/Revolut offers a more favourable GBP to USD rate? I'm aware that Monzo uses mastercard rates whilst Revolut uses their own rates
b) On Monzo, every time I do a transfer from GBP toUSD, I see an interim rate with the disclaimer 'this is not the final rate, it will settle after a few days'- does this mean they can change the amount they charge me if the rate 'after a few days' is different?
c) On revolut, one can create alerts for currency exchange rates- are you also able to set up a conversion order when the exchange rate hits your desired amount? i.e. if 1GBP=1.31, exchange xxxxGBP to USD.

Thanks!