Is MariaDB easier and better suited for a WordPress website?
1) There is very little difference between MySQL and MariaDB, especially for a beginner. Stick with whatever ships with your preferred Linux distribution.

2) You cannot do this even between different versions of MySQL and MariaDB.

2.1) You can only upgrade in-place, and not downgrade. If you need to downgrade, you will have to mysqldump and import on the new version.

2.2) If you are upgrading, MySQL and MariaDB versioning differs, so choose carefully if you intend to switch in-place. In some cases, upgrading requires special preparation, e.g. between MariaDB 10.1 and earlier / MySQL 5.7 and earlier to MariaDB 10.2 and later or very recent MySQL 8.0.x.

Bottom line - don't version hop. Pick one and stick with it until you know exactly what you are doing.

3) Yes, MariaDB speaks the same wire protocol as MySQL so anything that will run against one at a low level will run against the other.