Sanctions are not going to deter anyone from anything. Sanctions is anti-diplomacy.
Why do you need troops if you're not going to amass those? Still does not mean that diplomacy can't be useful.
The mental health argument is weird. Was Bush a madman when he invaded Iraq?
There're no more what ifs. The madman was amassing the troops on the border for months, dismissing any possibilities of preparing an invasion. Then invaded, and openly blackmailed the rest of the world with a nuclear strike, all spreaded by his chief "diplomat".
Present day russia as state proved to be pathologically incapable of maintaining signed treaties.
If anything, the diplomacy needed to rely on the effective sanctions. However, that's were the failure was. Since the 2014 the sanction pressure proved to be insignificant to alter russia's course. Did we get a second chance at it now?