instruction:
Given a travel related situation, and given a specific question about the situation, summarize the question into a title
question:
Background

I've recently entered a relationship with a person from Canada. Part of that commitment involves visiting frequently throughout the year; this commonly involves me driving to pick her up, as opposed to flying to save cost, so she can stay with me in the US. She's planning on coming up to visit again for the entirety of the Summer (four to five months).

Extra information in case it has any impact:  


She is 20 years old.
She is a third-year university student and is committed to her program for another two years.
She has visited the US three times already this year. Twice to visit me (~five weeks in total); once to visit her friend (five days).
We've both lived in our respective countries all our lives.
I've only stayed in Canada with her for a few days at a time.

My Knowledge
Feel free to edit this information if any of it is misleading/incorrect.

The 180-day limitation mentioned in these threads [ 1 ][ 2 ] are for people who visit in one consecutive stay; not for totaling 180 days in the year.
She will be considered a resident alien of the US if she crosses the 183-day mathematical formula, as noted on this AmericanLaw website:


  A foreign national will meet the Substantial Presence Test if he or she is present in the United States for 31 days in the current year and the sum of the following:  
  
  
sum of the days present in the United States in the current year,  
one-third of the number of days present in the first preceding year,  
one sixth of the days present in the second preceding year equals or exceeds 183 days.

  
  If a Canadian meets the Substantial Presence Test but is ineligible to meet the Closer Connection Exception (i.e. since he or she is present in the United States for more than 183 days in the current year), he or she will be considered to be a resident alien of the United States. Resident aliens are taxed on worldwide income in the same manner as United States citizens.


Question

Are there any common limitations, mainly regarding stay durations or frequencies, that could make further visits throughout the year difficult or impossible?


Also, I fully understand that a visitation could be rejected for many reasons and that there's no guarantees regardless of how long of stay. I'm simply trying to avoid breaking any rules or missing any legal obligations that can come up from this international relationship. This is all foreign to me— pun was most certainly intended.

answer:
What visitation limitations can come from a Canadian visiting the US for too long in the year?


question:
I've heard that in some overnight trains, rooms don't lock (no door); instead its just a curtain. This means thieves can get in and out easily. Is this true? If so, how do my friends and I keep ourselves and our belongings safe? (Even if we secured it with some sort of lock and cable, if we were sleeping, I would imagine that thieves could just open everything up.)

answer:
Overnight train luggage safety concerns in Europe


question:
I have made Sihanoukville, Cambodia, my travel-around-Asia operations-base for some time now. Now I need to travel to Europe and back... To save some cash on tickets, I will be going to the Thai border by bus, get a visa and fly with Finnair, then return back to Bangkok, catching a bus back to Sihanoukville.

It suddenly hit me that when returning I will be landing in Bangkok and it's my final destination. And I might be asked by Finnair when boarding for proof of onward travel even if I started my journey with them in Thailand. And usually bus tickets do not 'cut it'.

I have Lithuanian passport. 

Do I need to get proof of onward travel for my return flight to Thailand, even though I am using the same company (Finnair) to do the round trip to my original destination?

answer:

Going from Cambodia to Europe via Thailand and back. Do I need proof of onward travel for the return flight?