The PC-based DC in this case is particularly unsettling because it doesn't make sense that a caster won't get any better at it as time progresses.
That would be explainable, however, if you take the view that you're simply speeding up the ritual. Since by RAW all casters of any level take the same amount of time, then it makes sense that way. Unfortunately, that view breaks down in the proposed houserules by Dragonblade with the variable casting time. i.e. How do you decrease the time? By rolling well relatively speaking.
I will agree with you on awkwardness of the PC based DCs. Its purely a balance construct.
However, to make it a little more palatable, you can explain it as the caster having to gather external magic and focus it to cast a ritual. Because the ebb and flow of magic around them is not something a PC can control, the difficulty to gather the magic needed to cast the ritual will always have relatively the same difficulty regardless of the level of the PC. The roll the PC makes represents a combination of how much ambient magic happens to flow around them round by round (the Moderate DC) and their skill in gathering that energy (their skill bonus). Thus having the relevant skill trained as opposed to untrained, or having a high relevant ability score can help you roll better.
Can a PC stop a ritual if he rolls badly and then try again? For example, I imagine rolling badly and, say, getting 5 rounds. Oops, that's not going work, so stop the ritual and try again. Even if that wastes the first round action, you still might roll less than 4 (even a 4 makes you even so nothing lost). Does the component cost occur even if you spend 1/5 rounds? Does it occur if you're interrupted?
As stated in my rules, if a PC stops a ritual they already started casting, then the rules for interrupting a ritual apply normally per the PHB. In other words you don't expend any components or pay any costs unless the ritual succeeds.
Also, I think you might have misunderstood how the actual rolling to reduce time works. You don't roll once up front to determine how long the ritual will take in advance. You roll every round you are casting the ritual.
If you fail to beat the DC, that round you spent casting does not help you. The next round you beat the DC, so that counts as 1 minute off. The next round you beat the DC by 10 with a nice roll. Thats 3 minutes off. You are now on round 3. You have now effectively contributed 4 minutes to the casting time of the ritual. You can stop casting the ritual and lose nothing but the time already invested in it. But there is no way to determine how much longer its going to take to cast it and there is no advantage in starting over since you would lose the 4 minutes you have already accumulated towards completing the ritual. You just have to keep spending standard actions casting until you have effectively accumulated 10 minutes (assuming a 10 minute ritual) at which point the ritual takes effect.
Its a tradeoff decision the party has to make. Is having the ritual casting PC spend standard actions to continue to cast the ritual more important than using that standard action to use a power or to do something else?
If you think my rules are too generous than bump the DC to make it Hard. If you prefer PCs to have an easier time casting rituals in combat, then make the difficulty Easy. The DM could even change the difficulty depending on where the PCs are and what ritual they are casting to reflect high or low magic areas. If the PCs are casting Shadow Bridge in the Shadowfell, then the DC might be Easy. Casting it in the Feywild and the difficulty might be Hard.
I hope that addresses some of your concerns.
