Not acessing the site in a Multisite

Changing country in the front end cannot change from one site to another, you are just changing within the same site.

https://tennisreferrals.com/location/india/ is the country page for India of the main site.
It is not the same as https://tennisreferrals.com/india/ which is the homepage of a sub-site, which has currently no default location.

A multisite does not share anything from one site to another, only users are shared.
If you want to have different countries in different sites, you need to set each site to default country listings only, and add manual links to the other sub-sites.

Note also that if you use PayPal as a gateway, there is no need to set up for multiple currencies, that is handled by PayPal itself, depending on the PayPal user’s setting.

Also review this post: https://wpgeodirectory.com/payments-manager-v2-setup/
Payment Manager Version 1 is being retired and replaced with the new version.

Thanks

Note also that if you use PayPal as a gateway, there is no need to set up for multiple currencies, that is handled by PayPal itself, depending on the PayPal user’s setting.

So I am not sure why you need different sites for different currencies, but I will assume you do.

Ads are different for each country site

Maybe not a reason for multisite, but your call: https://wpgeodirectory.com/docs/useful-plugins/#logic

1. I presume one needs to setup each country site though it is a fresh site?
Correct

2. If you do not network activate any plugins then how will they be installed in the subdirectory?
You will need to activate them for each sub-site.

3. Any CSS in the main site will need to be redone?
Not if you use the same theme for each sub-site, and you add the CSS at the correct place.

4. how do you set each site to its default country?
First set a default location for the sub-site in the relevant country.
Then activate Location Manager.
Then go to GD > Location Manager > Location Settings tab > Country > Enable default country (country drop-down will not appear on add listing and location switcher). > YES
https://wpgeodirectory.com/docs/multilocations/#gd

Thanks

The price will always display as pounds, but once a user goes to PayPal, conversion should be shown.

Thanks

Each subsite can have its own admin, only superadmins can see all admin areas.
https://codex.wordpress.org/Roles_and_Capabilities

There should be no speed difference, and I doubt it would have any impact on SEO.

No, that is not possible. Admin has access to all the backend.