I'm sorry that you are having a lot of disappointments, but I will have to respectfully disagree with one of my buddies on this thread and say that a lot of what you list as 'negatives' could indeed have been avoided with some advance planning or reading. If you are still reading, here are some recaps of tips you were already given to hopefully improve the remainder of your trip and reduce the amount of what you say are 'negatives'.
Extra room: as stated on your other thread by many of us, the maximum occupancy in many, many (most) hotels around Europe is either 2 or 3 and that includes an infant. Booking a room for 3 and expecting to have a toddler as an 'extra person' won't work, and you will most likely have to pay for an extra room. Hopefully you did book that correctly for your other stays
Credit cards: again, earlier I noted that a North American credit card often does not work, especially in machines in France and in Italy. Hence I advised one to carry cash, and to carry more than one may in North America. I'm not certain if I translate your attempt to 'pay a premium to transact it' but if the machine or vendor won't take your North American card, it won't take the card. Offering the operator extra money won't work. Carry cash for these circumstances.
Shorts: shorts are not permitted in many churches around western Europe, but especially in parts of Italy, France, Austria, and Germany. Hence the recommendation to pack long pants
Cost of food: if you had brownies in Venice, it sounds as if you may have eaten at a place targeting tourists. (I see that OP is still writing EuroDollars, so I assume that he means that he spent around €5 for a small bottle of pop?) When I worked in Venice for extended periods I would often have a quick cheap lunch or dinner for under €7 consisting of a large slice of brick oven pizza and a crêpe or gelato for dessert. Eating local is alway cheaper than eating at tourist places. Hopefully you found a DeSpar in Rome to purchase 'pop'. Soft drinks are not as common in much of Europe as in North America, and water or wine is generally cheaper in restaurants. For anyone reading for future reference, the hint to find a supermarket in Venice doesn't work as well as they are few and far between. There are more smaller local speciality grocery stores, and a larger Austrian Billa a little further away from the main tourist areas.
Adaptor: you really cannot expect a hotel to provide one, and to be frustrated because one didn't have one when you didn't plan ahead really isn't fair. You could have purchased several at the Dollar store at home. If you didn't purchase a UK one, go to Boots as they sell them for North Americans who need a UK adaptor.
Extra seat: I'm not sure why you feel 'hassled' when the DLP site clearly states that if one doesn't purchase a ticket for a child under 3, a seat is not guaranteed. One really cannot expect to take a revenue seat for free during peak season, and that holds true for many venues, trains, etc. If you want the seat, you have to pay for it, or hope that it will stay empty.
Your time in Paris and Salzburg is something I will not comment on at the moment (as they were both my 'homes' during my life) other than to say that it sounds like you didn't plan for what you did end up seeing. Even on a Tuesday morning in November Versailles has queues, as does the Eiffel Tower. If you plan in advance, you will see more and find the best times for things. If you are doing any of the major tourist sights in London, do plan ahead for a better experience.
For anyone else reading, I strongly suggest avoiding the Sound of Music tour. At about €37 per person these days, it is a significant amount to pay for not much value, and the locals remains stymied and slightly amused why so many North Americans do this tour. Nor does one really see Salzburg; most of the movie sights and most of the must-see sights in Salzburg cannot be seen by bus, but rather on foot. It would have been especially charming in Salzburg as OP was there during festival season (I know that OP has no interest in that, but there is even more street performances, etc going on during festival)
I'm glad that you took my advice and listened about the dining discounts with the DLP AP. That advice I gave certainly saved you quite a bit of money on those events you list.
It sounds like you are doing a LOT of driving. Hopefully you are actually getting out to see the local sights and athmosphere.
London is quite empty in areas such as Oxford Street, etc although the TfL daily emails are still very detailed with changes to regular patterns. (Hopefully you signed up for those) The art galleries and museums are quite easy to get around now with low tourist figures, so I strongly suggest those. This week the main event location at Stratford may be busier than it was so far. And my two LHR experiences this past week were not good; far worse than the usual summer mess thanks to crowds, delays, inexperienced travellers, and large groups.
Enjoy the rest of the trip; hopefully you did have time to come back and read some of the advice to try and salvage the remainder of the trip and avoid some of the pitfalls you found to date. And hopefully you won't hold that against us here, as we did advise that one has to understand the differences between travel here and in North America. If one plans and understands and embraces those differences, it can be a wonderful holiday!