supposedly your 140 has been approved

EB2 filed 485:
1. What other documents related with employment will INS ask for after submiting 485 ( submited with employment letter)?

that's it. if you are not comfortable you can get another undated evl just in case

2. Will INS check for employment status before approving 458 again?

usually not, unless you get rfe or local interview

3. If the company closes down after I submit 485 within 180 days, am I totally screwed up or there could be a chance that INS might not know it?

not necessary unless you get rfe within 180 days.
if uscis does't touch or process your case within 180 days, then you are fine if you find another similar position.

4. If the company closes down after I submit 485, is there a chance I can use EAD to work or it's totally impossible?

yes you can work with ead for a similar position.
however, if cis process your case within 180 days and request an evl for your current employer, then your 485 will be denied.