1. Russia has an overwhelming military force going into Ukraine - how long can Ukraine hold out?
On their own, not long. Plus, Russia has been strategizing this for a long time and understands the military infrastructure of Ukraine because 30 years ago, it was their own military infrastructure too. Surveillance and espionage has only gotten better since then.
I'd be surprised if they last to March.
2. are there any 'allies' Ukraine can call upon to help in this fight? (my guess - NO)
No. Thats the whole reason for the fight. This is a long answer that I'll cut down because otherwise I'll sperg out and write a book. I'm REALLY cutting a lot out of a complicated situation. Like, a lot.
In the early 50s an agreement between a bunch of countries in Europe as well as the US and Canada was created to address the concerns that the Soviet Union would use their recently acquired infrastructure (German war reparation) and battle-seasoned troops to take some shit over. This was called NATO. Turkey became a member which was a big problem for Russia because the US could position missiles to hit Moscow from Turkey. More on the impact about this from later.
In the 1960s there was the Cuban missile crisis. We all know this story. A methed up JFK keeps missiles out of Cuba while clapping Marilyn Munroe's cheeks. Things remain tense with Russia.
In the 1970s the escalating fear of nuclear annihilation had the USSR and the US sign an agreement called SALT to keep advanced nuclear technology from glassing the planet. Subsequent agreements were signed because they needed to be updated and the agreements came with built-in expiry dates.
In the 1980s and 1990s the political landscape of the Eastern Bloc (USSR and their allies) changed. Ukraine emerged independent when the USSR broke up and signaled they wanted to have a better relationship with the west/Europe. Ukraine was NOT a fan of Russia because of many reasons, including a
genocide
This was a big problem for Russia but they were too disorganized and too poor to do anything about it. They famously had to sell submarines to Pepsi because they needed the money. They weren't doing shit about Ukraine but Ukraine wasn't doing shit either because they were dealing with their own problems.
Putin becomes the leader in the early 2000s, spends time building up the country like a mob boss (making a lot of people rich in the process), and in 2014 takes a strategically unique part of Ukraine (part as a flex, part as a test of the world's reaction. He succeeds and shit is tense.
The Trump presidency happens. Long story short; it was a very good time for Putin.
And this is where we really answer your question:
Russia has spent a lot of time isolating Ukraine from the people who would be able to support them. They have kept them out of the EU (which would have helped Europe and Ukraine immensely) and have kept them out of NATO (which would have helped NATO and hurt Russia). Had NATO brought Ukraine on board, Russia would have had a serious issue. Instead of being "adjacent" like Turkey was, Ukraine was a road trip away from Moscow. NATO would end any sort of Russian ambitions to attack anyone.
Russia has signaled, hinted, and all but promised they would nuke anyone who helped Ukraine. The SALT agreements I mentioned earlier have evolved and the current agreement (expiring in 2026) is called New START. This limits the amount of nuclear-capable weaponry to around 3000. There are probably more secret nukes but, hell, all you need is one. The desire to help Ukraine is there, but people really believe Putin would launch nukes so they're not gonna force his hand. This isn't like North Korea. Putin's shit works.
3. is there really a 'genocide' going on in those separatist regions that Putin is claiming and thus his reasoning for this attack?
No. That part of Ukraine is unique in the way that their loyalty leans to Russia. I'm over-over-over-over simplifying this in a very inaccurate way, but think of that part of Ukraine as being Quebec. Their loyalties are a little different than the rest of the country, but they're still "forced" to be Ukrainian. This is where Putin's statement of "genocide" comes from. It gets more complicated because of some military stuff, but the answer is no.
4. Sanctions - where is the proof that this concept works?
Simple answer: Kinda
Specific to Russia answer: Better than you might expect.
Russia's economy isn't great and its largely propped up by gas. They exert control over Europe using their pipelines which run through Ukraine. Shut off the payments and replace the gas sources, Russia gets rekt. Sidebar on the pipeline thing: check out "Occupied" on Netflix to get a taste of what might be in the future for Europe if Ukraine is taken without much resistance.
Its hard though, and would take a lot of time and effort. For now the things that can being done (wealth seizures of rich/influential Russians who may have Putin's ear as well as cutting Russia off from global banking infrastructure) are starting to be done. Russia will hurt in the short term but if they can rally, thats a problem. Complicated answer but its the best chance to minimize harm and prevent a large scale war.
I haven't even mentioned China, Belarus, Poland, and Hungary which are moving parts in this thing. Shit has gotten complicated. World War 1-level complicated which was its own clusterfuck.
tl;dr - shits fucked yo