[book cost fluctuation ????]

Seems IG takes 0.5% profit at sell and buy time and exchange rate fluctuates at sell and buy times too. I don't know if IG allow to keep your fund in USD , trade in USD and keep profit in USD at convert at your discretion to avoid losing at each trade.  My current understanding is , if we make 5% profit on a trade and USD/GBP fluctuate in your benefit by 1% you almost make 5% profit , since 1% IG conversion rate is covered but if exchange rate fluctuates against your benefit by 4% , your 5% profit is gone through 1% IG and 4% FX rate. IF you find out how to set multi currency , please let us know. 

[Can I open a USA dollar account?]

ISA has to stay in GBP , IG converts USD to GBP + 0.5%.  At least this is my understanding. .

[Extremely baffled on currency conversion fee]

0.5% of value at every exchange , sell side + buy side = 1% in the meantime , currency fluctuation risks , for example buy time - sell time = 4%  loss or gain , 1% for IG + 4% exchange loss = 5% of your trade value . If your profit is 6% in USD , your profit become 1% in GBP .  

[How to open a US dollar account?]

Hi 

That's a good questions.  As far as I know , you can put currency conversion on manual mode.