Software application copyright opinion

I would like to request your experience, if any, in regards of
copyright and trade secret laws specifically applicable to programming
community.

While I worked with an employer I was asked to write a proprietary
word editor application that would be used in-house. There are several
word editor applications out there, like Microsoft Word, Word Perfect.
There also are books and other resources available on the public
domain which tell what a word editor is what are it's essential parts
e.g. editing interface where users can type text, formatting, import,
export, save tools etc. etc.

There were two additional features in this word editor. One allows
users to enter a reference # and an existing acknowledgement letter is
pulled from company's business database. The user can then
view/edit/save/export it.

The second feature converts the displayed document into a JPEG file by
calling a third party text to JPEG library.

When I leave that employer, I decide that I will write a new word
editor and sell it under my name. I neither have access to the code
that I wrote nor possess it; I undertake coding of the new word
editor. There would be many similarities in both the applications
like:
1. Both run on the PC with Windows OS
2. Both have a user text entry area like MDI child form
3. Both will have a toolbox for formatting text, export, import or
saving the text the user just created.
4. Both applications use menus and the appearance of the application
of underlying user interfaces is based on Windows User Interface
guidelines published by Microsoft.
5. Both applications object oriented design (OOD) and try to follow
n-tier architecture.
6. Both will store the data on the hard disk.
7. Both applications are written by the same programmer

Both applications have dissimilarities as well:
1. The programming language is different.
2. The code is written right from the first line onwards.
3. Data storage format is different
4. New application does not have search by reference number module, as
this was specific to first application and thus employer's trade
secret. However, I do offer customization add on feature if my clients
request some kind of integration between the word editor and their
business data.

Besides the above there is a gray area, which constitutes the
following:
1. Since both applications are word editors, their user interfaces
have a lot in common as they both offer same main feature e.g. user
text entry.
2. Since both applications are word editors and follow OOD, there is
every possibility that many objects would have same (or maybe similar)
names. For example, I do not know what else should I call ‘Font'
object. By the same token the application structure could also be
similar because of similar functionality of both the applications. For
example, a word editor would have a MDI parent and several children in
presentation layer. And, in business layer, say objects like
application, document, font etc.
3. Since the same programmer writes both applications, the programmer
uses his/her collective experience (of which previous word editor is a
part), there is a possibility that an algorithm that was used
previously to convert the user entered text and format information
into disk write data bits is used from memory. Though the algorithm
was derived from the code sample that were provided by the development
language help or some user group samples or tutorials.
4. Both applications offer text to JPEG conversion using some third
party library. Can that be treated as trade secret?

My questions to the legal and programming community here are:
1. Can my previous employer claim it as copyright infringement?
2. Can any part of the application or the whole application be claimed
to be trade secret that I am not supposed to use anywhere outside?
3. What are your view/experiences on the gray area points that I have
mentioned above?
4. What would be the best way to develop the application legally?

RK