The "NO" on the union seemed emphatic - any reason for that? Are they working on one?

That's you reading too much into my answwer. Q: Union? A:NO, there is no reason as to add anything to it.
The answer to the last question is: Not to my knowledge.

How do international basing rules, pay, and benefits differ from domestic?
Outside of taxation diferences, the conditions are the same.

Any protections against being kept working instead of going home for off days?
Think Nancy Reagan. Just say no!
But. There is language on the GOM that allows them to extend you, although as far as I have seen or heard, when pilots tell them they can't, then that's it.
The reason being that since they offer 200%, somebody else will pick it up if you can't.

I noticed the 3 month bidding system - never seen one that far out... Is it possible to plan out 20-30 day off blocks?

You can bid back to back days off every other bidding period. You can also request to add your PTO to your days off by coordinating with them.
On the last couple of bids, they have published a couple of lines with lesser work days, but that will take you bellow Min. Guaranteed.
They seem very willing to work with you if you need extra days on one end and just work them at the other end. You only bid for days off, on your duty days you will experience a plethora of schedule changes. It's just the nature of the Ad-Hoc business model.

If you don't care at all where you go on your duty days, then the unpredictable nature of our schedules becomes interesting. But if you are the type that gets frustrated by schedule changes everytime you open your scheduling app, and then 3 more changes before you end up doing what's on your schedule. Then you need to look elsewhere.

This company has created a niche for themselves by responding to the costumer when they need it, they are very proud of the fact that they have placed a 747 to the other side of the planet 12 hours after a costumer has called. That means we are waking up in Frankfurt, thinking that we are headed to Hong Kong just to end up in Acra, at a grill house having dinner. That's life on the 74 fleet, understand what you're getting into.

Without a union, it can all change on a dime and for a reason of their choosing.

Yep. I am aware.

Imagine working for an airline with no union.... Big YIKES.

Worked for plenty of union (ALPA, Teamsters, and two different European unions) and non-union airlines throughout my career. Honestly, very little difference.

Thanks to the bar set by unions at other carriers, the market conditions, and the labor forces willingness to put up whatever without recourse.

On the "upswing" of a labor cycle non-union companies are forced to try to "keep up" with what others are offering simply because the small pool of people who refuse a union at any cost is too small to satisfy their labor needs, while a union shop may require more time to reach an agreement to improve pay and benefits in such a cycle. On the "downswing" of a cycle the non-union company can unilaterally reduce pay and benefits with little to no recourse by the employees, the union shop, on the other hand, can stick to the CBA they have and maintain a level of financial security for it's members. Unions are almost always the better choice for the common employee - hence why I would at least hope there is some degree of organization occuring, though doesn't sound like it from the (now) several people I've spoken with.

Unions are a better choice for work rule and salary improvement. But they don't really protect anyone during severe downturns. When it comes down to downsize and furlough, the seniority system and union schemes eat their young.

Some union shops set the bar pretty high, then there's us at FedEx where we are excavating below the basement to find a lower place to put the bar.

Ours is still lower than yours overall so keep up the good fight good sir! When groups do win it tends to bring up the others normally. There are outliers obviously along with the economy and management of course, but hopefully butts in seats requirement is restored at some places and if you're hemorrhaging pilots excessively than a more agreeable compensation package will be offered. Most things are cyclical, but I must admit the downwash you're experiencing is definitely a tactic we've experienced for over a decade and I wish you the best. Getting better at Giant, but I am cautiously optimistic.