Yup!

Use "javascript" on client site to do pinging. Or Java app running from web as applet. Or Flash.

There are two major problems - javascript doesn't support cross-platform post without hacks. And not all computers comes with Java. Same is with Flash.

TL;DR - YES

Long story - i'm author of similar desktop tool called SEOPingler:
http://www.mobiliodevelopment.com/seopingler/
so anyone you can use to ping anything. And bots are coming within second or two. This works perfect.

The problem is when you use this to ping many URLs (like 10k-20k). At some time this stop working and ping API endpoint receive your request but i can't see that bots are coming. This mean that there is some threshold that if you pass it for IP and you're temporary blacklisted. I also heard (but i can't confirm this) that this temporary may vary due previous usage. For me this isn't problem because users can blacklist their own IPs. And they can use hotspot wifi internet or VPN for continuing pinging.

But on server this will be HUGE problem because you can't switch IPs on-fly. And no one can guarantee how long your IP will be blacklisted.

Well - IMHO we are too deep in technology and know differences between multi-regional and multi-lingual sites. But when talking to someone "newbie" he easy can be confused with ton of terms. That's why they give example with something easy to be understanding.

Well there are 4 types of multi-lingual or multi-regional sites that are described here:
https://support.google.com/webmasters/answer/182192?hl=en
Summary - best is ccTLD, next is subdomain with gTLDS. On 3rd is subdirectory with gTLDS and URL parameters is last.

So Moz is trying to diving expert advise to webmasters to NOT use subdomains and giving famous blog example. I fall in this trap almost 15 years ago with subdomain. I wish someone to told me about this then... Other examples can be catalog and different products (catalog.example.com vs. example.com/catalog; product.example.com vs. example.com/product/). This advice in same subdomain keep link juice inside, but you probably know this.

GSC allow setting different directories to specific geo-location. That's true. But you can't change server location with subdirectories. With subdomain this is possible. Example - one company with site of Spanish and German. With subdirectory i have one server and /de and /es folders. But in this case server location is one and only. And server IP is some of signals for geo-targeting so you have tough choice where to be. With subdomain you can make de.example.com and place this in German server and es.example.com and place this in Spanish server.

That's why subdomains for multilingual sites is notable exception of golden rule "do not use subdomains".

This is discussed many times. And answer is YES. You can see here what Josh Bachynski says:
http://themoralconcept.net/pandalist.html
Look on #5 in section Low quality factors.

That's why you can see here or in blog sections moderators often remove links to sites. Because some people just make comment to get a link to their site. You should do this too. Or make them "nofollow" at least.

Quick way to remove staging url is sending HTTP error 410 as result.
Other is to use in SearchConsole Remove URLs function https://www.google.com/webmasters/tools/url-removal

About duplicate content - you must see actual canonical. If on stage URL there is canonical point to normal site then you shouldn't hesitating. But if staging and normal point to different URLs then you can see some algo filter.

TL;DR - Yes, you should use both for better CTR in SERP and Social Networks.

OpenGraph is described here:
http://ogp.me/
and OG is mostly used from Facebook and G+.

Twitter used TwitterCards:
https://dev.twitter.com/cards/getting-started#opengraph only for them.

G+ used shema.org or OG:
https://developers.google.com/+/web/snippet/ 
https://developers.google.com/+/web/snippet/article-rendering

Pinterest used RichPins:
https://developers.pinterest.com/docs/rich-pins/overview/

So you should read excellent article from Cyrus Shepard about all meta tags:
https://moz.com/blog/meta-data-templates-123
where everything is described with examples.

And answer is YES! Today is 2015 and within few days will be 2016. Today shares in social networks are so much important than also baclkinging. Of course organic visits and visits from social networks are different, so you need to implement both of tags to get maximum performance there.

That's correct.

Long story - there is hreflang in HTML head and in xml sitemap. What's the difference? In HTML you can use same-site or cross-site links (between domains), in XML they are only in same domain. So in XML sitemap you can't use cross-site links (between different domains).

Also in SearchConsole you can set "international targeting" (SearchConsole -> Search Traffic -> International Targeting -> Country) for different sites.

I know that hreflang is little bit complicated but once you setup your first site correct then on next sites will be much faster.

1. Look perfect. But i'm not sure that you can index in site X pages about site Y even in hreflang. You need to make test on some site. Please note - there is cross site canonical tag and this is ok. There is cross site alternate hreflang and this is ok. But i don't know about sitemap with links to other sites even in hreflang. You need to verify this in SearchConsole - upload sitemap and see for some warnings or errors there. Also don't forget that setup hreflang in HTML code too -> https://sites.google.com/site/webmasterhelpforum/en/faq-internationalisation#q16 https://moz.com/learn/seo/hreflang-tag https://moz.com/blog/hreflang-behaviour-insights

2. Answer of this dependent from "validation" of previous question - can you setup sitemap of site X with links to site Y. IMHO you can't. So you must put sitemap about int.com to int.com site and usa.com on same site. Like your example and this is for usa.com site:
<url><loc>https://www.websiteUSA.com/</loc></url>

3. Yes, should be.
"Hreflang tags may not help you increase traffic; instead, the goal of using them is to serve the right content to the right users. They help search engines swap the correct version of the page into the SERP based on a user's location and language preferences. For information on how geotargeting can help with rankings, learn about ccTLDs."
https://moz.com/learn/seo/hreflang-tag

4. Can you show me this? Because your sitemap may miss XSL for showing proper. Or you may return different HTTP content type.

Seems as related to AJAX crawler escaped fragment.

You should see why bot is trying to crawl normal pages with deprecated method. Please see "incoming links" to this 404 in SearchConsole to see where is root of this problem and fix it.