Update: A major rework after understanding the question better (see
comments above).
Plain TeX keeps the right sequence for each insertion class as stated
on page 122 of the TeXbook:
Each class of insertions is independent, but \TeX preserves the
order of insertions within a class.
There is only one insertion class for floats: \topinsert. All
pageinserts are topinserts and midinserts are implemented using either
topinsert or they use direct output. Thus topinsert and pageinserts
are always in the right order but midinserts might get out of
sequence.
For example, in your example Figure 1 (midinsert) is inserted after
the first bsptext. Then you request a topinsert, i.e., a figure that
has to be placed at the top of the current page or if that is not
possible at the top of the next page. Here it is possible so it starts
the current page. The next topinsert is placed after the first because
it fits too. This mix gives inserts out of order.
The TeXbook describes the general procedure in chapter 15 on pages
115--116 (dangerous bend paragraphs); look at the example above
exercise 15.5 and at this exercise for an example with topinserts and
a single midinsert. The general procedure how inserts are added to
pages is given on page 123 (double dangerous bend paragraphs).
On page 363 of the TeXbook you find the code. Look at the definition
of \endinsert to see the conversion of midinserts: The first \if@mid
checks if the insert fits on the page; if not @midfalse is set and
the second \if@mid outputs a topinsert.
Conclusions:
(1) Midinserts and topinserts cannot be used in parallel without the risk
of an out-of-order sequence;
(2) using only topinserts and pageinserts guarantees to keep the order.
(3) A new insertion class could be defined that places the floats not
on the top of pages if (2) is not liked. In this class the order is
kept. But this means also a change to the output routine as it must
handle the insertions.
(4) A new command similar to midinserts can be defined that outputs
floats either directly or converts them to topinserts and forces after
the first conversion for the rest of the page additional midinserts to
topinserts and topinserts to midinserts. This does not require a change
to the output routine.
An approach for (3) can be found in this TUGboat article by David Salomon:
http://www.tug.org/TUGboat/tb11-4/tb30salomon.pdf; see page 593ff.
How to implement (4)? A couple of \ifs are set if a new
midinsert-like command is processed, let's call it seqinsert. If it is
placed in the text \seqinsertplacedtrue is executed which will convert
topinserts for the page. If a seqinsert is converted to a topinsert
\blockseqinserts is set and all following seqinserts on the page are
converted to topinserts too. Both flags are reset when the page number
is increased; this happens in the macro \advancepageno.
\newif\ifitisaseqinsert % true: a seqinsert is processed (used in \endinsert)
\newif\ifseqinsertplaced % true: a seqinsert was output as midinsert
\newif\ifblockseqinserts % true: a seqinsert was moved to the next page as topinsert
\def\seqinsert{\itisaseqinserttrue\midinsert}% extended \midinsert (not a \newinsert !)
\catcode`@=11
% harmless changes to \endinsert; no problem with existing plain
\def\endinsert{\egroup % finish the \vbox
\ifseqinsertplaced \if@mid\else \@midtrue\fi\fi % new (A)
\if@mid
\ifitisaseqinsert \ifblockseqinserts \@midfalse\p@gefalse\fi\fi % new (B)
\dimen@\ht\z@ \advance\dimen@\dp\z@ \advance\dimen@12\p@
\advance\dimen@\pagetotal \advance\dimen@-\pageshrink
\ifdim\dimen@>\pagegoal\@midfalse\p@gefalse
\ifitisaseqinsert \global\blockseqinsertstrue % new (C)
\global\seqinsertplacedfalse\fi % new
\fi\fi
\if@mid \bigskip\box\z@\bigbreak
\ifitisaseqinsert \global\seqinsertplacedtrue\fi % new (D)
\else\insert\topins{\penalty100 % floating insertion
\splittopskip\z@skip
\splitmaxdepth\maxdimen \floatingpenalty\z@
\ifp@ge \dimen@\dp\z@
\vbox to\vsize{\unvbox\z@\kern-\dimen@}% depth is zero
\else \box\z@\nobreak\bigskip\fi}\fi\endgroup
\itisaseqinsertfalse} % new (E)
\catcode`@=12
% minor addition for the output routine
\let\orgadvancepageno=\advancepageno
\def\advancepageno{\orgadvancepageno
\global\blockseqinsertsfalse\global\seqinsertplacedfalse}% reset flags for every page
Here are the explanations of the changes to \endinsert:
(A) converts topinserts to midinserts if one seqinsert was not converted to a topinsert
(B) a seqinsert was converted to a topinsert so the next is converted too
(C) here the flag for the conversion seqinsert to topinsert is set (and the flag for
(D) is reset to avoid overhead by (A))
(D) here the flag for a non-converted seqinsert is set
(E) reset the flag that shows that seqinsert was called
If this code is added to the example topinserts, pageinserts, and
seqinserts can be used without getting out of sequence; the command
\midinsert might destroy the sequence and should not be used.
For example, add the above code and change your output to:
\bsptext
\topinsert \mypicture \makefigno \endinsert
\seqinsert \mypicture \makefigno \endinsert
\topinsert \mypicture \makefigno \endinsert
\seqinsert \mypicture \makefigno \endinsert
\topinsert \mypicture \makefigno \endinsert
\seqinsert \mypicture \makefigno \endinsert
\seqinsert \mypicture \makefigno \endinsert
\bsptext
Note, I never used this code; I created it to answer the question.
