From reading all the information you have provided it sounds like your 500 has serious issues.
If you turn it on and it does nothing, no color flashes etc and you have reseated all your chips then there are problems. You can start off trying to swap chips if you have them on hand, if you do this start with the 68000 IMO and then Agnes(just my quick guess). But personally I think you'd be better off buying a replacement. If mine are too expensive for your taste, used but working units can be picked up cheap on eBay. Since you have everything else just look for an auction for a A500 console only(no monitor or even power suplly). You can probably pick one up for $20 or so + shipping.
HERE is one at $9.99 (NOT MY AUCTION!) with 1 day left. Assuming you are in the USA of course.
