Bike Fit & Bike Size

In my opinion I would not worry too much about a bike fit if you are riding a bike under $1.5K.
As long as you are comfy and pain free as your bike.
In regard to Size.... fest ride the h, to and See how it feels. minor adjustments can be made with different stems and different dip-on aerobars

You already know if your bike fits or not. It's a road bike so there's not quite as demanding a fitting process. They may very well shim a cleat or both and probably will fuss with your seat position quite a bit.

Get a tri bike fitting before you buy a tri bike. There's a world of difference in fittings here and getting it slightly wrong can be the difference between a comfortable ride and trying to peddle while straddling a razor blade for 3 hours. The fitter will suggest a frame size as well. This is a much different fitting than a road bike fit.

Price has not one thing to do with whether a fitting is worth it. I've sold off a couple bikes for under $1K and would suggest the $200-300 fitting even for those.

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Fit is the most important no matter what bike it is. But, for a flat bar road bike... there's only so much that you can do to change the fit, so I'm not too certain that makes sense. I bet in the near future, you are going to want a "proper" road bike... I would get fitted now and the use that data to purchase a bike in the future (and you should always get fitted BEFORE buying...).

Here's the problem with your second question... There's no standard to what a size denomination means, nor is there a standard how they are measured. In essence, a 56 from one brand might be similar in fit to a 54 from another, a 57 from a third, medium from a 4th, large from a 5th and so on... Then add to the fact that you are comparing a tri bike to a road bike and there's no relevance at all.

Can you make a bike that's too small work? Yes, but only with quite a few compromises to fit. There's 3 parameters, comfort, efficiency and aerodynamics. A good fitter will find the perfect balance between the 3 based on you (i.e. body dimensions, core strength, flexibility, etc.), your athletic experience/background, terrain, distances and types of racing/riding, prior/current injuries/issues, etc. etc. etc. If you start off on a bike that doesn't really fit well, then you have to compromise even further just to make it work, but that can never be as good as starting off with a properly fitting bike... Fit is by far the most important aspect of purchasing a tri bike.

Sometime ago Tom Demerly outlined the difference between a fit and an adjustment.
From what I recall a fit normally costs between $250 and $500 and should be done once a year at the beginning of the season. The intention is to optimize not only your position to be as aero as possible when on the bike, but also maximize the power output.

Then you can look at an adjustment, this is where the bike shop looks at how you sit on your bike an advises on things such as saddle height, stem, etc.
Such an adjustment does not have you pedaling and measuring power output in the process.

IMO, a bike adjustment does not need to be done annually as your body physic does not change annually. The purpose is to get you comfortable on a bike.
I would not spend around $300 on a fit annually when I ride a sub $1.5k bike. Just because it seems to me that the ratio is off.

I would say that reliability (everything works and doesn't brake down on you) is the most important thing. This is given on a new bike, but not on a used bike.
As no one will finish a race if the bike breaks down....

I can fix that on a used bike, but I can't change the basic dimensions of the bike...