Hi Matt,
Do you already have experience starting from seed? If so, start these plants as you normally prefer and the keep potting them up until they are in the size container you want for your centerpiece. By July they will be full and in bloom with no forcing necessary. If you don't have experience starting seeds, then you'll need a little more information from us ...

I'm not sure how to answer your question #3, since I don't know what size your peat pots are. a 5" pot is *slightly* larger than I'd use for direct from its germination tray, but it's not too big. however, I'd make sure those babies had a good root system before you put them in the final pot, otherwise the soil won't hold any water and they'll probably get stressed trying to establish themselves. It'll be for more than 2 weeks after germination. personally, I don't start anything in peat pots anymore unless they don't transplant well (like nasturtium, for instance!), but you have to go with what you personally have had success with.
5" isn't too small for the root systems exactly... it will be by august and september, but if you're timing it so the plants look fresh and proportinate to the pot for july, then it'll be ok. i know it's a galvanized pot, but consider whether having those pots in your gh, getting watered, in the elements, etc. will discolor the pots. they may not be too pretty for a centerpiece after a few months of that. I don't have a good suggestion though, unless you can find a plastic or fiber pot to use as a liner (ie, grow in the other pot, then on that weekend pop the whole thing into the decorative pot. I have 4.5" square pots, so I know they exist, but I can't say the same for 5" square pots.

so some extra tips: nasturtiums prefer nutrient poor soil to flower well, otherwise you'll get more leaves than flowers. what type of chrysanthemums were you planning on growing? unless you're good at timing blooms, I'd go with plants that continuously flower, otherwise the mums could bloom at the wrong time.

for your favors, skip the pansies and violas. they'll look awful by a MD July with the heat. trust me, i'm here too. substitute impatiens, vinca, begonia or geranium. geranium are really easy from seed and grow faster than the others. petunias are great, but kind of sticky. 3" pots are pretty small, so start the plants a little later than most guides would say.

Truly your inside space would need to be artificially lit,sunflowers do not like to be transplanted..I think it would be simpler for you to buy pony packs in April and pot them in to your centerpiece pots..FWIW..I'd find a cheap plastic pot to plant them in that fits inside your tin pail for a centerpiece.Then you do not have to worry about dents or stains on your pails..
I'd guess you are going for trying to minimize expenses...but starting from seed is tricky..and IMHO you do not need the added stress of trying to get this just right on your 1st try for a special day. Frankly i'd find a brides maid, MOH, or Mother of the groom... or someone who's got a lot of plant experience to do this for you.No matter which way..from seeds or pony packs...you go.
Have a lovely weeding and enjoy your marriage.
the above is all free advice...so take what you will from it.

The only way to succeed is to grow a ton of them and hope that some of them are in bloom on the magic day. Sunflowers are the only thing on your list that would be remotely reliable for a July wedding and they are one of the trickiest to get to bloom on schedule. Nasturtiums are a spring flower that fail in high heat and mums are a fall flower that don't initiate flower buds until the days start to shorten. But there are plenty of blooming plants you could get to bloom easily. If you need 12 table top plants, sow 24 seeds the first week (110 days before the wedding date), two seeds per pot; then sow 24 more each week afterwards until you get in the 60-70 day range. Sunflowers can grow fast or slow depending on the weather and the care you provide. A million things affect their blooming - so rather than try to control all of that, just sow a new crop each week and hopefully one group will be in full bloom by wedding day. Oh, and if you want only one plant per pot (a good idea) you need to murder one seedling after they have sprouted. You plant two per pot in case one of them doesn't sprout. They develop new varieties every year so check the seed packet to see the days til bloom (the large sunflowers are around 110 days with a spring planting, much less with a summer planting). There might be new types with a shorter time frame.

Sunflowers need bright light to be happy. Raising them indoors is very difficult - because of the light issue.

Once they start to open up they can last a week. If they appear to be a day or so ahead of schedule you can move them into a very cool room at night - or pray for cool temps outdoors. Cooling them off (even if it is just at night) will slow them down a couple of days.

Pansies and violas can be tricky if spring is hot and muggy, they start to go downhill when things warm up.

Impatiens may not have the perfect name for a wedding flower but the blooms are pretty reliable and they grow easily from seed (sow them in March for July bloom though).

It is far easier and cheaper to buy small blooming plants a week before the wedding and feed and water them well.

I can give you a personal experience about how finicky plants (even ones already grown) can be....so starting from seed would seem about like making a bet in Vegas...I purchased several dozen full grown stargazer lillies for my wedding (they were going to go on the steps of the church in really nice old world urns (I purchased these full of buds in February for my March 5th wedding) My plan was that they might be in full bloom but even so the bloom would still last and look and smell wonderful for the big day...one week before the wedding, still no open flowers, the count down began. I started giving them hot water baths and created a pseudo sauna in my master bathroom, hoping to force the blooms...3 days and counting...still NOTHING, NOT ONE flower. 2 days, same thing...day before the wedding, I had to run out and find a replacement, which I did find and it all worked out...I'd say about 2 or 3 weeks AFTER the wedding, these lilly's decided to bloom....I had over a dozen of them in my garage and boy oh boy what a fragerance.

Moral of my story...(if it's not obvious) I had my heart set on these plants, they were fully grown, full of buds and I thought it was a sure thing...the plants had a different clock going.

You can certainly trim your budget in more important areas like food, beverages, etc. I'd try to find a reliable florist and even order your cut flowers so that you can put together your own table pieces. I even had a good friend make my bouquet and the brides maid bouquets and the mens' flowers, etc. It saved a fortune. We used whatever flowers that were left over for the reception tables.

I had a second reception in Idaho with my family in June, I brought with me the same little tin bucket thingies and cut roses from my mom's yard and made my own center pieces for that event too. They were simple and gorgeous and fragerant and FREE. And everyone liked them.

You will have plenty of other things to spend your energy and worrying over, flowers would NOT be one of them for me. There are cheaper ways to get what you want. Maybe you even have access to a flower market or growers market? Even a decent farmers market?

I guess I would finally say, if you guys really have your heart set on doing it this way, Have a good back up plan that you can rely on at the last minute.