Should I choose IBKR's tiered or fixed pricing, and what order size keeps brokerage low?

Yes, better to place a larger order to optimise on brokerage (reach min charges).

HDFC, ICICI, Axis -> All have the same deduction as shown in the Paasa app. What you see is what you get; no extra remittance/swift charges.

a) Yes, Tiered is the cheapest for small orders like 500,1000,1500 USD single orders.
b) I would pick a USD-denominated one in case it shows multiple exchanges. Charges are mostly the same across all European exchanges trading in USD.

Please choose tiered in your IBKR account settings. I learnt it hard way when 4USD for the order was charged first time.

Fixed is only useful when you are placing large amount order, when 0.05% of trade value is more than 4USD. That would only exceed when you are placing a single order of more than 8000 USD. All orders below that tiered would be cheaper.

Yes, you can buy fractional shares. In case total quantity in an order is less than 1, say 0.3 shares, then both tiered and fixed have same cost of 1.7 USD. But when total shares bought/sold are more than 1, tiered would have minimum 1.7USD charge, fixed would have 4USD charge. So its safer to set Tiered in account settings always unless you are placing high value orders.