There is only a few of these and AFAIK they all have a dual ID number; one IC and one RE. They are essentially IC trains that take local tickets on part of their route.
Indeed. When you're looking at the route planner (eg bahn.com), if the same train (same timing, same calling points) is listed as both an RE and an IC then you have one of those elusive combi-trains that are best avoided if travelling on a Deutschlandticket. And if you forget to check, then:
they are all IC train sets in DB white+red stripe.
so you can easily spot them on the platform.
And as regards the cancelling the subscription, AIUI, if you buy a Deutschlandticket after the tenth of the month for that same month then you're signed up to at least 2 months worth, because you can't cancel the second month in time.
However, a bus company in Sachsen Anhalt allows you to pause the subscription right up to 24h before the end of the month (ie the penultimate day).
I haven't found any other source for the Deutschlandticket that allows this, and I wouldn't be surprised if some busybody or other doesn't drag the bus company to court for some stupid reason, but in the meantime, you can use it on your phone or laptop, or even print out a piece of paper (presumably with a QR-type code printed on it?).
https://www.mopla.solutions/de/tickets/deutschlandticket
Edit to add: you can pay by credit card (worth trying whether it works with a debit card) or SEPA (like a direct debit) from a bank account (will this work with a UK bank account, even though we have IBANs attached to our accounts?)