In terms of connectivity, yes: you should not meet a screen telling you the data is gone, and you should not be billed more. In terms of speed, no. Almost every such plan sets a full-speed amount per cycle, and once you pass it the speed is reduced until the next cycle. The figures and conditions differ between plans, so they have to be read on the plan's own detail page.

It depends on the plan, because the reduced tier differs between plans and some do not state a figure at all. The measure that helps regardless is downloading the map area you will be moving through while you still have full speed or while you are on Wi-Fi, so navigation is not depending on a live connection for every tile.

The most common explanation is that you have passed the full-speed allowance for that cycle, which is a normal term of this kind of plan rather than a fault. The other is congestion on that network at that place and time. Before troubleshooting, read the plan's usage terms and look at the data figures recorded for that profile on your handset.

It has to be checked plan by plan, because some permit it and some do not, and the rule varies by provider. Where it is permitted, shared traffic normally counts against the same full-speed allowance, so it runs down faster than phone-only use. Read that line on the detail page before buying if a laptop is part of your plan.

Not always. If you use little data and spend most of the trip within reach of Wi-Fi, a fixed allowance is usually cheaper and often goes unused. If you navigate all day, take meetings on the move, or carry the hotspot for a group, unlimited earns its price. The most direct way to decide is to read your real consumption from your handset's own usage screen and hold it against the full-speed allowance.