Crossing the Cambodia/Vietnam border and single/multi entry visas

My girlfriend (Austrian) and I (Spanish) are going soon to Vietnam/Cambodia for holidays. When we booked the flight tickets, a quick research in terms of visa showed that she needed a visa for both countries and I only needed to get a visa for Cambodia.

However, things aren't so easy all the time...

The last few weeks, after researching and reading through embassies, forums and so on, certain doubts and confusions started to arise.

On the way there, we will be flying from London to Ha Noi directly. On the way back, from Phnom Penh to London, via Ho Chi Minh (Stop there for around 3 hours before departure).

The first issue is how to enter Cambodia from Vietnam and how to get the visa in accordance.

We originally thought there might be a train that connects both countries but this isn't the case. Then we thought with a flight from Ho Chi Minh but this option exceeds the budget. At last we were left with the bus option.

According to forums and travel websites like tripadvisor, lonely planet and the like, bus border crossing can be problematic due to scams.

In any case, regardless of the mean of transportation, we need to get a tourist visa for Cambodia. However, in the visa form document from the Cambodian embassy in London we must specify what the entry point in the country is. We have asked the embassy with no response as of yet.

How can we know the exact entry point if we take the bus option? Is the entry point the destination or the specific place at the border where the guards will ask for our passport? How bad/dangerous the scams are at his point?

The second problem is on the way back to London. My passport allows me to enter Vietnam only once every 30 days. Is the flight connection from Phnom Penh to Ho Chi Minh considered a second entry? Again, we have read that both Cambodian/Vietnamese guards can make us go through immigration several times at this point. Is this a common procedure? Is it a randomly decision by the guards or all foreigners must follow it?

Finally, given said possibility; I suppose my Spanish passport only won't allow me this and might have trouble at the airport the second time around. I would have to ask at the Vietnamese embassy for a multi-entry tourist visa, is that correct? (I have also asked this but no response as of yet).

I have never been in a situation before where between connecting flights I have to go through security once more, hence my confusion.

Sorry if the topic is duplicate, I have done a search on the site but couldn't find relevant information. I also don't have links handy right now to show some of the info mentioned in the post. If required I can try to search my browsing history to provide them.

Apologies for the length of the post and thanks a lot for your help and time!

Cheers,