政府、1日75万人の感染者想定し対策
Is it equivalent to 政府、一日七十五万人の感染者想定した対策?
What is the grammar behind this pattern?
Is 一日(に) abbreviated?
Sometimes 連用形 can end a clause without て. The construction is called 連用中止(形). For example, see the following questions (you can search others on the site).
I assume it is a headline or something. Just like in English, headlines try to be succinct and often omit particles.
1日(に/あたり) are rather variants, as X a day/X per day in English.
A fuller version (not unique, of course) is
政府は1日(に/あたり)75万人の感染者を想定して対策を検討している
Semantically 想定した対策 works perfectly. But omitting た in a noun modifying clause is generally not possible.
FYI Some other questions on headlines.