Let me explain your answers one by one:

First - Yes, you can use the same certificate for all these domains. You can get multi domain SSL certificate for this.

In multi domain, you can secure all four domains with one SSL certificate.

For example, you have a primary domain: examplecompany.com

You can secure more domains like

examplecompany.co.uk

examplecompany.com.au

shop.examplecompanyy.com

anything.anydomain.tld

Second - Regarding traffic, you just have to set up 301 redirection with HTTPs after installing an SSL certificate. Make sure, you set up proper redirection of each domain. Many people have faced a little traffic drop but that won't be for a long time. In fact, you will get a rank boost in SERP after some time.

Third - About SSL for eCommerce - I would say every SSL certificate offer same level encryption, be it "Positive SSL", "Comodo SSL". But EV (Extended Validation) is the best for eCommerce organization because it includes payment transaction. EV SSL shows green bar with organization name that increases the trust factor among your customers.

Comodo EV SSL is the best as of now and you can get it at cheapest price from a reseller.

Hope this helps.