That's about 11 years and he still wants to remain in power by changing the constitution and he just arrested his biggest opponent who would have won the next election which is the reason for the massive across this NATO member state these days

Constitution is losing its power and value, or perhaps it has lots a long time ago. If it's possible to change constitution at any time you wish and how you wish, then what's the point of the constitution? Putin changed constitution the way he wanted, the package of constitutional changes included an amendment allowing him to run again for the presidency in 2024 and he can do it again in 2030 and stay still 2036. The president of Belarus, Lukashenko, abused constitution the way he wanted, that was really extreme).
Deploying NATO or UN troops on Ukrainian territory would not be easy. This would depend on the terms of the peace treaty and as far as I know, Russia is against this idea, they would never accept a peace treaty that included this condition. France, England, Germany...want to do it but that doesn't mean they can do it because they don't have the authority to decide that.
Like before, Britain made a bold statement and was ready to send troops to Ukraine, but later, British Armed Forces Minister Luke Pollard corrected that: they would only do so with the support and participation of the United States. If the United States refuses and does not participate, it will never happen.
Whoever offered this to Putin, seems to be a very sarcastic person.