These are all great tips that I also try to use, so I'm only adding on from my own experience. I've had to travel a lot for work and have found myself in situations where I don't end up getting much say on the food (served by a conference venue or the restaurant is chosen by another employee or client). To my pleasant surprise, I've found a lot of places willing to help make adjustments even if that option isn't on the menu. For example, some pasta dishes may be able to add grilled veggies instead the usual shrimp or even use oil instead of cream-based sauces. I've even had restaurants make up a full veggie plate of their various vegetable sides. Just remember to be kind and not come off as entitled or judgmental, and people will help you out.
When I do have the chance to provide an opinion with a group, I try to suggest cuisines that tend to offer vegan menu items by nature (a lot of different Asian and African cuisines offer non-dairy and non-meat options, even having designated sections of the menu to highlight this). This is usually more acceptable to others than suggesting a 'vegan restaurant' and typically doesn't draw any attention to the vegan aspect one way or another.
I find when people try to 'go vegan' they can get too hung up on replacing foods they can no longer have (especially with vegan substitutes and eating at strictly vegan restaurants). The most sustainable way to become a healthy vegan is to focus on what you get to eat. Becoming vegan opened up doors for me to try new vegetables and cooking approaches I never would have before. And that's what was important for me—not to simply cut out the bad but bring in more of the good.
And in the end, be flexible and allow yourself some room to not always be 100% perfect all of the time. There will inevitably be butter in there somewhere, and unless you have an allergy, you'll be ok.