Best thing to do is rent a car for your entire time in Scotland. If you rent for blocks of days within your two week visit, you're going to end up paying a lot more than if you rent for the entire two weeks. Petrol is expensive, but cars in Scotland get excellent fuel mileage. The exchange rate right now is $1.22 to the pound. Hopefully it will be in that range next April.
The cheapest places to buy petrol are in in the forecourts of the major supermarkets - Tesco, ASDA, Sainsbury's, Morrison's, Co-op, etc. If there are two supermarkets near one another, they frequently have a price war, which would be to your advantage. At the current price of petrol in the Glasgow area (1.13 UKL per litre), and approximately 4 (3.78) litres per US gallon, you'd be paying about $5.25 for the equivalent of one US gallon, which is not that much different from what you would pay for premium unleaded in California.
Unless you plan to drive the North Coast 500, or head for some of the remoter corners of the Highlands and Islands, you will find public toilets everywhere. In addition to regular public conveniences, supermarkets, pubs, hotels, department stores, museums, and restaurants all have toilets available to the public. In some of the more remote parts of Scotland, you may be asked to contribute a small amount of money if you use the toilet in a shop or restaurant. Sort of like spending 50p to spend a penny! :) Mrs. Auchterless and I have found toilet facilities everywhere in Scotland (although they are limited on the Small Isles, St. Kilda, and Handa). At one point, we had considered writing a "Good Loo" guide to Scotland for those, like us, who have frequent urges.
If you hire a car, as opposed to taking public transportation, your opportunity for exploration will be increased tenfold. The most beautiful parts of Scotland are virtually impossible to reach by train and bus. In the Highlands and Islands, the places that you want to see may be only served by bus service once or twice a day. Also, with two of you travelling, your rail/bus costs would be doubled. Plus, with a car, you don't have to carry your luggage everywhere.
You can hire a car for your two weeks, and still take a day trip (or two) with Rabbie's, or another tour company. As you don't get to stop for long at each place you visit, a tour would whet your appetite for getting back to your car and spending more time at places you visited.
If you have a Visa/MasterCard, you can use it in lieu of car insurance. Your credit card will reimburse you for any damages to your hired car. You pay for the damage when you return the car, and then submit the paperwork to Visa or MasterCard. It takes a while to get reimbursed, but the system works. You have to decline all coverages (except towing, if you decide to accept it) offered by the rental company in order to activate this coverage.
Don't fret about driving on the left. It takes about an hour to get used to. On the motorway, the slow lane is the left lane, not the right lane. If you're at all apprehensive, hire a car with an automatic transmission. It's one less thing to worry about. The roads in the Highlands are narrow, and even in some of the smaller towns as well. So driving in Scotland does require a little more concentration than you would give in your home town. Traffic circles (roundabouts) can be a bit unnerving at first, but if you read up on the rules and procedures before you head over, you'll be fine. As with driving anywhere, a little courtesy goes a long way.
Very best wishes for your April holiday. The daffodils and crocuses will be in bloom while you're over, and newborn lambs will be frolicking on the hills in most of southern Scotland.
@rmg643 - You can't drop a hired car off on Skye. The nearest drop off point for any of the major rental companies would be in Inverness.