Very good, glad to see you doing some research. 
Yes it is valid. When the flex-wrap value is omitted it is set to the default value.
Initial value as each of the properties of the shorthand:
Yes you can. I prefer to use shorthand so I can make adjustments to one or both properties later in a media query if I need to.
Here’s some good reading for you on that. It’s an old article but the specs haven’t changed much (if any) on inline formatting. Don’t get too involved in it, but it will be good for you to see that it is more complex than most people are aware of.
Inline formatting model
It’s just not a good idea to rely on inline format for laying out blocks in your page.
In fact, the display:flex on the parent <div class="view"> wasn’t really doing anything but stacking your divs as blocks, which is how divs layout on their own anyway.
Anytime you use an anchor for any type of box formatting it’s normally best to go ahead and set it as display:block; such as with what your doing or in the case of links in a navbar. If it is just a link within a string of text then yes it would stay at it’s default of inline.
Here is an example of what was going on when your anchor was left to it’s default of inline. Why it was working, and the other strangeness that was going on with it. You’ll see how the box breaks and wraps to a new line but only extends as far as it’s content. Perfect for text but not for page layout.
<!DOCTYPE HTML>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Inline Box</title>
<style>
div {
max-width:600px;
margin:auto;
border:1px solid;
padding:10px;
box-sizing: border-box;
}
div * {box-sizing: inherit;}
a {
display: inline; /*default*/
border: 2px dashed red;
padding: 4px;
text-decoration: none;
}
img, span {
display:inline; /*default*/
vertical-align:baseline; /*default*/
line-height:1.2;
}
img {
width:100%;
max-width:400px;
height:auto;
}
a span {
padding:5px;
background:yellow;
}
hr {
margin:20px 0;
}
h1 {text-align:center}
</style>
</head>
<body>
<h1>Inline Anchor Box</h1>
<div>
<a href="#">
<img src="https://via.placeholder.com/400x225" alt="" width="400" height="225" title="www.project.com">
<span>www.project.com</span>
</a>
<hr>
<span style="border: 1px solid red">
Lorem ipsum dolor sit amet consectetuer in faucibus malesuada Vivamus nunc.
Cursus id tempus enim semper ante congue Vestibulum Nam pretium lacinia.
</span>
</div>
</body>
</html>
