Answer: The answer to your questions depends on which plants you are trying to grow. If April 20th is your last frost date, then April 10th is really a bit late to be starting most things indoors except possibly tomatoes and fast growing annuals such as zinnias or cleome.
You might do as well to plant them directly in the ground where they are to grow, or save the seeds to plant next year when there is more time.
The main reasons for this are that most plants need the early spring when the temperatures are mild and the soil is moist in order to grow a strong root system. This system will support them when the season turns hot later. The seedling/transplant process also requires about a week or so during which the plants are gradually acclimated to the out of doors, this is done to try to reduce "transplant shock"; if the weather is too warm, then the risk of shock is greater. In my experience, direct seeded plants are at less risk for shock.
Tiny transplants can be set out successfully, but may require more care initially because they are small and it is somewhat difficult to handle them without damaging them. Having said all that, if you are willing to experiment a little bit, it is always worth a try with plants listed at being started at up to six weeks before frost. For plants where the suggested timing is ten weeks, it is really too late for this year.
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