Please find below answers to your questions based on my knowledge.
If I get endorsed for global talent will my 2.5 years be counted for ILR ?
In general yes it will be counted towards ILR. Depending on which route you’re applying to, the years it takes for you to obtain ILR differs. If you’re applying to exceptional promise, you will need another 2.5 years. If exceptional talent instead, you only need 0.5 more year to apply for ILR. (if you’ve been staying in UK long enough every year to meet ILR requirements)
Do I need to pay IHS even though I have a valid visa for 2.5 years?
Yes, you will always need to pay IHS to complete your new visa application. If you’re applying to exceptional promise and it’s successful, you will get 2.5 yrs refund as you’ve paid it twice. The exceptional talent scenario could be a tricky, I’d suggest you double check.
Facing same issue here. the annoying part is that, one reputable lawyer said the opposite but asked a high fee (which I was ok to go with) - but the two other said I couldn’t combine these times. Any progress on your side @Burak_Uzun ? (hopefully )