So you can say what 'will work' but no one can say what 'won't work.' Yeah... not for real. TA was jam packed, you can't just add 3 people in there and everything be "just fine." Where would their screentime come from?
Some of this Ant-Man "logic" is actually a bit silly. Wasp and Ant-Man aren't attached at the hip. They aren't any more a package deal than IM and War Machine. And making an Ant-Man movie and not putting him into Avengers makes perfect sense in this context, where Ant-Man isn't available and isn't needed. But something compels Ant-Man fans to state he is an absolute necessity, and that his concept is inherently captivating to all, despite clear evidence to the contrary. It's a strange sort of entitlement that seems proclaims itself as logic. Certainly you aren't the only one doing this, but since you brought it up as something no one has place to question, I just wanted to point out how very questionable it is.
And Marvel does want to do Ant-Man... as a standalone thing. As you said, they'll find a way to make it work, despite the naysayers, as well meaning as they may be.
I don't think you can disassociate Ant-Man from the Avengers that easily. If Marvel is making an Ant-Man movie, and all signs point to the fact that they are, then it will certainly be a part of the MCU. Ant-Man and Wasp are very much a package deal as she will be in his solo movie and then brought along for TA2.