Hello, I will help where I can. I have lived in Bratislava for 2 years now, in many different parts of the city, and have certainly experienced some fun things as a foreigner here.
1) I found Bratislava safe. There are the few drunks here or there, but nothing unsafe. Truthfully, the most unsafe I felt here was from a group of British tourist drinking here for a stag party. It didn't help that I was an American female, I think the Slovak females get it even worse though.
2)English is common-ish. In the main city you can get by with basic communication English. Nothing too advance. Going to the hospitals can be tricky. I would recommend finding Slovak friends quickly
As far as the market, if they cannot do numbers etc. Then writing is always an option. For hotels, the further East you get the less English is available.
3)Saving 1600/2500 after taxes, rent, food, etc. Depends on where you are living/renting from. You can find a basic 2 room flat in Petrzalka for around 300-400. It may not be the most comfortable or near to where you work, but it would help with the budget. Then food, I typically count for eating out + groceries around 170E a week for 2 of us. If you buy alcohol that price goes up. Keep in mind the visa process will set you back some money as well.
4)flat for 450 with all included, sure, but you will be looking at staying outside of the city center and sacrificing some comfort. But, I have personally found the flat rent trend to be curious, often fluctuating monthly. In other words, sometimes I see great flats for for great prices, other days I see terrible flats going for too much money.
Good luck on your travels!