It sounds like you want to use this web application as a remote control for your robot, and a core issue is that you won't want a page reload every time you perform an action, in which case, the last link you posted answers your problem.
I think you may be misunderstanding a few things about Flask. For one, you can't nest multiple functions in a single route. You're not making a set of functions available for a particular route, you're defining the one specific thing the server will do when that route is called.
With that in mind, you would be able to solve your problem with a page reload by changing your app.py to look more like this:
from flask import Flask, render_template, Response, request, redirect, url_for
app = Flask(__name__)
@app.route("/")
def index():
return render_template('index.html')
@app.route("/forward/", methods=['POST'])
def move_forward():
#Moving forward code
forward_message = "Moving Forward..."
return render_template('index.html', forward_message=forward_message);
Then in your html, use this:
<form action="/forward/" method="post">
<button name="forwardBtn" type="submit">Forward</button>
</form>
...To execute your moving forward code. And include this:
{{ forward_message }}
... where you want the moving forward message to appear on your template.
This will cause your page to reload, which is inevitable without using AJAX and Javascript.