Basically not really viable. You'd have to do all or nothing.
Personally, I think if such a "all-out-war" scenario is happening. Ramstein airbase, and any US airbase in Europe used for resupply or take-off of heavy bombers are all up for grabs or French Naval base in Toulon and other areas that involve resupply and recon, I think their is a large recon base in the UK that receives SAT information. It would only be a logical thing to do from Iran's perspective and all of it is very possible and can even be prepared today with the current technology available. Hence I've always thought most threats to be bullshit, since only diplomacy is the only option. Frankly, the IRGC has humbled West Europe. Such a conflict would be a surprise for many.
It is not like Iran is escalating anyways, in a all-out-war, they will try to level every piece of infrastructure in Iran that has military or duel-purpose use, setting the country back 40 years or so. They will use thousands of long range missile till they run out of stock. They will not spare any building or collateral damage, so why return these luxuries to them.
If Americans violates the Iranians airspace to bomb/send missiles inside Iran killing any personnel or civilians, Iran has legally the rights to strike inside the US homeland using any conventional weapons if that strike was also conventional, but also the locations from where these fighters taken off/missiles launched
Example! If American fighters take off from Bahrein, Iran has not only the right to strike this base in Bahrein, but also US homeland using proportional and conventional force
US homeland being hit would be the biggest nightmare USA would live since its creation, its people will not have the same thoughts that they are 100% protected from the invincible US army and that US homeland is a legitimate target if their soldiers continues to invade other countries, step a foot in any country without authorizations/killing people inside
From a realistic point, i think if Iran manages to strike US homeland if the US begins a war, it would result in an immediate nuclear blackmail from the US, like they did many times in the past (Against China, Iraq etc), and western armies have a casualty threshold since WW2 trauma, but the USA case is more complex since the 9/11 terror attack, if an Iranian missile strike a military factory inside the US during an all-out war, this would not be a "terrorist attack" but a retaliation, that's called an eye for an eye and USA and particularly its people never experienced this
