My programmer recommends that replacing ad is a better option. So in simple terms, the proxy will detect ad's and replace it with our ad. It is as good as putting a proxy to filter malware/virus etc. Still confused with the ethical part of the idea.
You have posted an interesting question. The way you ask it, sounds very bad, and very unethical.
However, It is common for proxies to have ads at the top of the page. I never see anyone upset about this practise at all. I do not see this as 'theft' or making money on peoples content, but rather your site is providing a valid service, and the ads are their to fund that service. So IMO, nothing wrong with this.
As for the second bit, alot of people do use ad-blocker. So I see no great harm in removing ad code for sites. Additionally, I own multiple sites, if you ran this and stripped out all the ad codes from my site, I would not care. No doubt traffic to my site via your service would be low. Besides, I didnt build my sites out of greed so it doesnt matter to me. But there are alot of very greedy people out there, and they would be upset by this.
As for your ads, I would say to keep it as a header at the top of the page, however, doing so, (without the perfect integrating into the site into optimal areas) will give you a much lower click rate, and much lower earnings.
Doing both, header ad, and stripping out ads from the site, I see nothing overly wrong with this.
However, I think its wrong to put your ads inside of the site. Like if you have a header ad, with the site displaying underneath, that kinda makes it clear, that that ad belongs to you, then I am ok with that. Kinda crosses a line when you hide ads into a site that isnt yours...
Whatever you do, do not use adsense for this. It will be only a matter of time before your adsense is perma-banned. You might not agree with this statement, however, it is your responsibility to ensure your adsense code is on high quality sites. A site that is banned from adsense, might still have the code on the site. Your site might then replace the banned code with yours, and BOOM! your adsense is perma-banned for being on a banned site that did not meet the adsense quality guidelines.
Remember, everyone only gets 1 adsense account in their lifetime. Be very careful what you do with it.
It should work fine. Just check the Google TOS to make sure this is allowed and maybe consider just putting ads at the top or side of the page instead if possible. It's possible you could run into contract issues there. I wouldn't worry too much about the "ethics" of it, internet ads are all crap anyways.