If I solve A and B in a good amount of time. Then if I try to solve C and fail at it once, would I have received higher rating delta if I did not attempt C?

For example, in the latest educational round, it says: //The penalty for each incorrect submission until the submission with a full solution is 10 minutes.

Does this mean that I simply have time deducted? Like if I had 40 minutes remaining, now I only have 30 remaining. And when I submit the correct submission, I am back to 40 minutes. Is that how it works? And if I get time deducted, does that mean that if I solve another problem (e.g. D) without solving C, I will have worse rating, but if I solve C first then solve D, it would be as if I did not do a wrong submission? Also, does that mean that if I keep C unsolved and stop trying in contest, my rating is not affected?

How are things different in regular div2 contests, if at all?

Sorry, I tried to google but could not find answer and I don't have cp friends (xd).

i think it's like icpc rule where extra time is deducted as points called penalty in case of wrong solution or late submission. And the penalty is then subtracted from the total scores of the problems which hurts your rating.

In educational rounds, if two people solve the same number of problems, the person with the lower total solve time across all solved problems will be placed better in the ranking. The time penalty is added to your total solve time — no time is deducted from the duration of the contest.

It's worth noting that if you don't solve a problem, the time penalty of any submissions don't count. So, in your first example, you won't lose anything if you attempt C and fail.

In regular div2 contests the scoring is much simpler — each problem has a point value that decreases over time, and for every wrong submission to a solved problem you lose 50 more points.