Because flights during the Christmas period are very expensive (especially when multiplied x4 for the whole family) we intend to drive, take a lot of toys / clothes to be donated and have the car to visit family in various parts of the country.

We need to ensure smooth crossing across the channel.

Because it will be during the Christmas period - increased traffic - potential snowfall - I'm planning for contingencies.

Eurotunnel:

https://www.eurotunnel.com/uk/tickets/

Basic logic dictates to purchase 2x return ticket and skip the return. After reading T&C:

EDIT / UPDATE: Let's see if there are some virtual credit cards providers out there: https://softwarerecs.stackexchange.com/questions/37332/is-there-a-software-allowing-me-to-generate-virtual-credit-cards-on-the-fly

Ferries:

Also - what happens if I select 10:00 ferry departure and I miss the boat? Am I automatically embarked for the next one?

What if I arrive 2 hours before the planned departure - will I be taken onboard?

I've never done this journey before, there are so many options and providers... I just want to ensure smooth sailing, without necessity of reading the small print from 10 different companies.

Your 'common sense' and 'common practice' advice is greatly appreciated!

Related: How can I cross the English channel with a car?

one often can get on an earlier train

EDIT / UPDATE: I'm based in the UK. I'm driving to Poland. Returning after Xmas / New Year. I want to know what are my options and pay a fair price - sometimes comfort (and peace of mind) is more important than being penny wise and pound foolish.

I've only traveled by ferry, because I found the Eurotunnel too expensive just for 15 minutes less of a trip.and I prefer floating boats over under-the-sea small train tunnels.

I'll quote the Dover to Calais ferries website:

If I miss my sailing, can I get on another ferry?
Yes you can, if we’ve got room and your ticket type is flexible. Check out the terms and conditions, paragraph 4iii) and our Which Ticket? Page for the details.

that paragraph:

What will I be charged at port for not travelling as booked?

£20 amendment fee
+ re-price

and about the arriving 2 hours ealier thing:

i. We can only guarantee availability including that of passengers,
vehicle and extras, on your booked departure dates and times.
Additional surcharges may apply for travel at other times. Refunds
will be subject to ticket type.

My brother used the tunnel recently, managed to check in a few minutes late for the one direction and with a few minutes spare in the other direction.

When late he was told he could no longer use the train he had booked, the next was completely booked but he was put on the next, so one hour after the one he had booked for.

If it had been very busy all day, he would still have been put on the first available train, which might have been one late in the evening instead of his missed morning one.

He was also told that the price you paid for the ticket booked is the price you get to cross, even if you go from a lower to a higher price time.

Most ferries have a like policy, but not all.
I am not familiar enough with the different ferries to be sure where you can 'miss' your slot and be put on the next available and where a missed slot will make you pay again.

When you check prices, do check all, as often the prices depend on how many others have booked for that same crossing/time, more than on the average price for that crossing.