Moving Existing domain + Existing G-suite email

Site URL: http://www.eneri.net

I am not a programmer and I have minimal knowledge, so I apologize if I use the terms wrong and causes confusion!

I have an existing website, eneri.net, currently hosting with myhosting.com, domain registered and hosted at Network Solutions, and existing email is hosted at G-Suite. I need to move my web hosting over to squarespace, the email also needs to work.

I have chatted with the SS customer support and I only got a bunch of links to documentation, they are not very clear on instructing what I need to do and if all this can work.  The only thing I learned from the chat was, theoretically it will work. My questions here:

- SS recommends that we "transfer" (instead of just "connecting/pointing the domain to the website") the domain hosting completely over to SS. Can someone really explain why ? Do we HAVE TO do that in order to gain access to the MX records, so I change it to point to Google/G-suite? 

- Will customer service help the migration when the time comes? So far I can only find support via chat and they copy and paste documents that doesn't sooth my concerns. I want to see if MORE technical support is available hidden somewhere.

- How long will my email service be interrupted from the time I have to move the domain / transfer the domain, to changing the MX records ? Any smart ways to avoid downtime to a minimal? I care less about my website being down, but email is crucial. 

- Can someone list out a step by step list on what I need to do for the most seamless migration? Has anyone done this before and care to share the dos and don'ts, tips and tricks?

- I don't want to "transfer" my domain hosting from Network Solutions to SS because I worry some years down the line I'll have to move web hosting, and it will be a pain or may not even be possible to get my domain name out. Is my worry valid? 

- Lastly an unrelated question - do we have to get the $18 business plan in order to use CSS?! Really?

Thanks in advance and I hope I will get help here.

I'll take a crack at the unrelated question. Site wide CSS should be available at all plan levels. However some Page types have an Advanced > Page Header Code Injection feature.. That is only available with the Business Plan or higher. So if you need the later then yes you'd need to go with the Business plan.