or that саде is still the prepositional case but is written саду if it referrers to a location?
This. "говорить о саде" but "находиться в саду". For most nouns there wouldn't be any difference (for example "говорить о цирке" and "находиться в цирке") so in general it's considered to be the same case, i.e. preposotional, but some nouns retain their old forms for locations.
Много requires that whatever noun follows it, is in the genitive case. The word following много may be singular (много воды, a lot of water (water is uncountable)) or plural (много кошек, много тарелок, много столов, a lot of cats, plates, tables, etc.).
In this situation, only the plural of flowers would make sense. I hope this is helpful to you. Please let me know if it is not clear or does not answer your question.
The form of the masculine noun in the prepositional with the ending in -e is characterized as a book form, in -у(-ю) form - as colloquial-professional, sometimes with a touch of vernacular. For example
в порте - в порту ( in port )
в чае - в чаю (in tea)
It still depends on the use of the word in a literal or figurative sense. For example
номер на доме - работа на дому (number on the house - work at home)
We are missing the тут or the здесь in the Russian to be able to use that for a direct translation of this particular sentence. However, if you wanted to, you could say that in English and it wouldn't be outlandish, just mildly redundant. You would probably say it while standing "here" in the garden, pointing around to all the flowers.
In English, "a lot of" is generally plural, because it encompasses plural objects (when used with countable objects). Flowers are countable.
Compare these sentences, which use "a lot of x are"...:
- A lot of people are in the room.
- A lot of shoes are in the closet.
- A lot of plants are on the window sill.
...to these sentences, which use "there are":
- There are a lot of people in the room.
- There are a lot of shoes in the closet.
- There are a lot of plants on the window sill.
(In spoken English, we often say "There is a lot of people," and only the most stringent at grammar will point it out, and they really need to get a new hobby; I know, because I used to be one of them; I got a new hobby! Anyway, anyone would be perfectly understood using this verbiage.)
Non-countable objects when used with "a lot of" will generally use singular. I cannot think of an exception at this time. See the below:
- There is a lot of time for thinking at my new job.
- There is a lot of gum stuck to the bottom of my shoe.
- There is a lot of water on the floor.